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Staphylococcus aureus: A pathogen with still unresolved issues
Infection, Genetics and Evolution xxx (2013) xxx–xxx
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Staphylococcus aureus: A pathogen with still unresolved issues Jean-Philippe Rasigade, François Vandenesch ⇑ UMR U1111 INSERM Université de Lyon, Site Laënnec, 8 rue Guillaume Paradin, 69372 Lyon, France Hospices Civils de Lyon, Groupement Hospitalier Est, 69 Boulevard Pinel, 69677 Bron, France
a r t i c l e
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Article history: Received 1 August 2013 Received in revised form 21 August 2013 Accepted 22 August 2013 Available online xxxx Keywords: Staphylococcus aureus MRSA Molecular epidemiology Bacterial toxins Gene expression regulation Vaccine strategy
a b s t r a c t Staphylococcus aureus is a major human pathogen, and considerable research efforts have been put forward to improve our understanding of its complex pathogenesis. In spite of these efforts, the burden of staphylococcal infections is still on the rise. This review focuses on a selected set of crucial unresolved questions regarding this pathogen, namely: (i) the nature of the driving forces behind the rise and decline of methicillin-resistant S. aureus (MRSA) clones; (ii) the mechanisms by which a commensal becomes a pathogen; (iii) the molecular underpinnings of toxin overexpression in hypervirulent MRSA clones such as USA300; and (iv) the repeated failures of anti-S. aureus vaccine approaches. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Staphylococcus aureus is one of the most important human pathogens both in the hospital and the community (Lowy, 1998). Since the beginning of the antibiotics era, S. aureus has been a privileged target of therapeutics research, and numerous antimicrobial agents, as well as vaccine attempts, were specifically designed to combat this bacterium (Lowy, 2003; Proctor, 2012). The overall amount of research efforts aimed at understanding S. aureus is still on the rise: searching the PubMed database with the ‘‘S. aureus’’ keywords over the 2007–2012 period retrieved 21060 articles, a 51.4% increase as compared to the 13907 articles published during the 2002–2007 period. Such efforts in both the fields of basic science and clinical research could have been expected, from a clinical standpoint, to result in dramatic improvements in cure rates of S. aureus-infected patients as well as in a global decrease of the incidence of S. aureus infections. However, the history of the last decades clearly shows that these expectations were not met. In 2005, methicillin-resistant S. aureus infection has become a more common cause of death in US patients than HIV infection (Centers for Disease Control and Prevention, 2005; Klevens et al., 2007). Besides this striking US-specific figure lies a more global threat: the increase of MRSA infection incidence in several regions of the world, not only in the hospital but also in the community (Boucher and Corey, 2008). Although our understanding of the lifestyle and the pathogenic strategies of S. aureus is constantly increasing, several important questions have remained unresolved. This review will ⇑ Corresponding author. Tel.: +33 472357252. E-mail address: [email protected] (F. Vandenesch).
focus on these questions, which could be major obstacles against our efforts to translate the scientific achievements on S. aureus into clinical successes.
2. The rise and decline of MRSA clones MRSA is resistant to all currently available beta-lactam antibiotics. Although this pathogen emerged rapidly after the introduction of methicillin in clinical practice in the early 1960s, it only became a public health threat during the 1980s. Its expansion was accompanied by the acquisition of resistance determinants to non-betalactam antibiotics, which led some authors to suggest that betalactam resistance alone was not sufficient to promote MRSA dissemination in the hospital (Knight et al., 2012). The spread of MRSA worldwide was due to a limited number of successful clones as determined using molecular typing techniques such as pulsedfield gel electrophoresis (PFGE) or multi-locus sequence typing (MLST) (Maiden et al., 1998; Mcdougal et al., 2003). MRSA is polyphyletic, meaning that the MRSA clones responsible for the past and current epidemics do not share a common MRSA ancestor. Indeed, MRSA emerged from methicillin-susceptible S. aureus (MSSA) through the horizontal transfer of a mobile genetic element, the staphylococcal cassette chromosome mec (SCCmec). Contrasting with previous studies favoring a clonal origin of methicillin resistance in S. aureus (Kreiswirth et al., 1993), more recent phylogenetic studies indicated that SCCmec acquisition was a frequent event in S. aureus evolution (Nubel et al., 2008), with MRSA subgroups such as ST36 or ST80 originating from genetically distinct MSSA lineages (Deurenberg et al., 2007; Rasigade et al., 2010). A
1567-1348/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.meegid.2013.08.018
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
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consequence of this polyphyletic nature of MRSA is that it can hardly be studied as a whole because different MRSA lineages often retain the different genotypic and phenotypic traits of their respective MSSA ancestors, and acquire additional features that reflect their specific evolutionary history. As a consequence, MRSA clones show striking differences in terms of fitness, virulence and the spectrum of resistance (Diep et al., 2006; Li et al., 2010). MRSA emergence and spread in the hospital and more recently in the community, is the result of consecutive and often intermingled epidemic waves due to different clones. Early MRSA strains belonged to the ST250 lineage, which is part of the highly successful clonal complex (CC) 8. This Archaic clone virtually disappeared from hospital in the late 1970s and was gradually replaced by other MRSA lineages. Some of these new lineages were genetically related to the Archaic clone but harbored different allotypes of the SCCmec resistance determinant, such as the ST247 Iberian clone or the ST239 EMRSA-1 clone, while some other lineages were genetically unrelated to CC8, such as the ST5 USA100 clone or the ST36 EMRSA-16 clone. The molecular epidemiology of MRSA has remained extremely dynamic, and dramatic changes in the relative abundance of each clone have occurred over the last two decades with, for instance, the replacement of the ST36 EMRSA-16 clone by the ST22 EMRSA-15 clone as the major cause of MRSA infection in UK hospitals (Wyllie et al., 2011; Knight et al., 2012), or the emergence of highly epidemic clones of community-acquired (CA-) MRSA such as ST8 USA300 in the US, which is currently replacing the ST5 USA100 clone in the hospital setting (Gonzalez et al., 2006; Maree et al., 2007; Patel et al., 2008). A thorough understanding of the driving forces that rule the expansion and decline of these different clones is clearly needed to help improve infection control strategies and contain the MRSA epidemics. However, our knowledge of these driving forces is still limited. The most obvious contributing factor to MRSA dynamics is antibiotics use and overuse. Nevertheless, several other important factors likely contribute to MRSA epidemics. For instance, high rates of MRSA have been recorded from remote populations with limited access to the healthcare system or antibiotics prescription (Mcdonald et al., 2006), thus indicating that antibiotics selection pressure is not sufficient to explain MRSA emergence. Similarly, infection control enforcement do not always correlate with the decline of a given MRSA clone: in the UK, the decline of the ST36 EMRSA-16 lineage appeared to have begun before that new infection control policies took place (Wyllie et al., 2011). During the 1990s, the decline in France of the gentamicin-resistant MRSA clone, which was replaced by the gentamicin-susceptible MRSA Lyon clone, was not explained by a reduction of gentamicin consumption (Lelievre et al., 1999), but maybe by a differential fitness of the two lineages (Laurent et al., 2001). Indeed, several clone-specific features could be linked with the emergence or decline of a peculiar lineage. Bacterial competition could occur between clones that share the same niche, similar to what has been demonstrated for S. aureus and S. epidermidis (Iwase et al., 2010). As mentioned above (Laurent et al., 2001), differences in bacterial fitness such as those observed between the highly successful ST22 EMRSA-15 clone and the currently declining CC30 MRSA are also potentially involved (Knight et al., 2012). Alternatively, the acquisition of immunity in the community in contact with a given clone could play a role in the subsequent decline of this clone (Gupta et al., 1998). Finally, lineage-specific features such as chlorhexidine resistance have been shown to influence MRSA dynamics in settings where infection control guidelines encourage the use of this antiseptic (Batra et al., 2010; Edgeworth, 2011). Overall, MRSA dynamics appear to be ruled by a complex interplay between human interventions, bacterial evolution and clone-specific biological behaviors, the consequence of which being our current inability to predict the epidemic potential of newly emerging clones.
3. What turns commensal strains into invasive strains? S. aureus is carried without damage in 30% of humans, mostly in the nose but also in the throat, gut and skin folds (Sivaraman et al., 2009; Faden et al., 2010; Yang et al., 2010). Carriage increases the risk of developing a staphylococcal infection, and healthcare-associated infections are usually caused by the strains carried in the patient nose (Luzar et al., 1990; Kluytmans et al., 1995; Von Eiff et al., 2001; Wertheim et al., 2004). An important yet still unanswered question is whether genotypic differences exist between harmless carriage isolates and those that cause invasive infections. Several molecular typing studies using either MLST or amplified fragment length polymorphism failed to demonstrate such a difference (Day et al., 2001, 2002; Melles et al., 2004), and there is no evidence that lineages exist that are specifically associated with either colonization or disease. Indeed, although the presence of specific genes has been undoubtedly associated with specific clinical entities, such as the exfoliative toxins with staphylococcal scalded skin syndrome, or superantigens such as TSST-1 with toxic shock syndromes, isolates harboring such toxins are also found in healthy carriers (Mccormick et al., 2001; Nishifuji et al., 2008; Lozano et al., 2011; Piechowicz et al., 2011). Another unanswered question is whether S. aureus colonization is beneficial or detrimental to the carrier. On the one hand, as stated above, colonized sites can act as a reservoir for healthcareassociated infections in compromised patients. Carriage is thus righteously considered detrimental in patients at risk for staphylococcal nosocomial infection, and decolonization measures have proven efficient at reducing the risk of staphylococcal infection in colonized patients undergoing hemodialysis or surgery (Tacconelli et al., 2003; Bode et al., 2010). On the other hand, S. aureus carriers that develop S. aureus bacteremia have a lower risk of death than non-carriers (Wertheim et al., 2004), suggesting that S. aureus colonization can induce some form of immunological adaptation beneficial for the host in terms of outcome if an invasive infection occurs.
4. Toxin expression regulation in S. aureus The molecular basis of virulence in S. aureus has been an active field of research for several decades, but such research has further intensified and become decisive with the recent emergence of highly pathogenic CA-MRSA strains that combined antibiotics resistance, rapid spreading ability and exceptional virulence (Chambers, 2005). The last decade has seen the identification of yet unrecognized S. aureus virulence factors such as the phenol-soluble modulins (Wang et al., 2007), as well as the characterization of the pathogenic role of long-known toxins such as the PantonValentine leukocidin (Gillet et al., 2002; Labandeira-Rey et al., 2007; Diep et al., 2010). Vast amounts of work have been necessary to gain understanding of the complex multifactorial nature of CAMRSA virulence and to outline which genetic features make one S. aureus strain more virulent than another. A clear distinction has been made between, on the one hand, virulence gained through the acquisition of new toxin genes by horizontal transfer such as the phage-borne pvl genes, and on the other hand, virulence gained through the overexpression of core genome-encoded toxins such as PSMs or alpha-toxin (Wang et al., 2007; Li et al., 2010; Otto, 2010). An additional layer of complexity has also added to this already intricate scheme with the observation that the acquisition of new genes harbored by mobile genetic elements (MGEs) correlated with modifications in the expression level of core genome-encoded toxin genes (Kaito et al., 2008, 2011). Gaining a deeper understanding of the molecular pathogenic mechanisms of CA-MRSA is crucial as a prerequisite to the design of dedicated therapeutic strategies,
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
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including those aimed at targeting specific toxins such as PVL or superantigens (Yanagisawa et al., 2007; Rouzic et al., 2010; Gillet et al., 2011). Moreover, lessons learned from CA-MRSA virulence research could help understand virulence in MSSA, which are still the main cause of severe community-acquired staphylococcal disease in Europe (Dohin et al., 2007; Gillet et al., 2011). In spite of these research efforts, several important questions remain open with respect to CA-MRSA virulence and, more generally, S. aureus lineage-specific virulence. First, the genetic features that lie behind the ability of the strains of a given lineage to stably integrate a given MGE are largely unknown. There is strong evidence that MGEs such as SCCmec cassettes or the PVL-encoding phages only integrate in a subset of S. aureus lineages (Vandenesch et al., 2003; Tristan et al., 2007), and both these MGEs seem to preferentially integrate into the same lineages (Rasigade et al., 2010). Lineage-specific differences in the defenses against foreign DNA such as restriction-modification systems (Corvaglia et al., 2010; Schijffelen et al., 2010) could tentatively account for the observed associations of specific MGEs with specific lineages, but this hypothesis warrants further research. The second question is that of the genetic features that account for lineage-specific differences in toxin gene expression levels. Recent reports have underlined the decisive role of the major regulators agr, sarA and saeRS in the increased virulence of CA-MRSA strains (Montgomery et al., 2008, 2010; Li et al., 2010; Kobayashi et al., 2011; Zielinska et al., 2011). Such regulators are expressed at higher levels in CA-MRSA strains such as USA300 as compared to other S. aureus lineages (Montgomery et al., 2008, 2010). Regulator overexpression, by upregulating toxin transcription and secretion, provides a comprehensive explanation to CA-MRSA virulence along with the presence of specific toxin genes such as pvl (Otto, 2010). However, the mechanisms by which such regulators are overexpressed in some specific S. aureus lineages are still unknown, and the discovery of these mechanisms will probably be one of the major challenges of the ‘‘omics’’ era.
5. The repeated failures of S.aureus vaccines One the most decisive advances in our struggle against bacterial diseases results from the development of vaccines effective at preventing serious conditions due to, e.g., Neisseria meningitidis, Haemophilus influenzae type b or Streptococcus pneumoniae (Seale and Finn, 2011). All these vaccines were aimed at eliciting protective immunization through a specific antibody response. Several approaches to induce a protective immunity against S. aureus have been made (Proctor, 2012). Understandably, all these vaccine candidates comprised selected antigens targeting humoral immunity; all showed promising results in pre-clinical animal models, but all these vaccine strategies failed in clinical trials (Daum and Spellberg, 2012). What went wrong? Understanding the differences between S. aureus and the bacteria against which successful vaccines were obtained is a crucial issue to help develop effective S. aureus vaccines in the future. First, pathogens for which vaccine attempts were successful had well-characterized pathogenic mechanisms, linked to a limited number of surface-exposed epitopes or secreted toxins. The multiplicity and frequent redundancy of S. aureus virulence factors, as well as the broad spectrum of diseases caused by this pathogen (Lowy, 1998), could help explain why the optimal antigen or combination of antigens has eluded us so far. S. aureus lives with humans as normal flora, the consequence of which being that virtually all humans possess anti-S. aureus antibodies (Verkaik et al., 2010). Hence, antibody-based vaccine attempts in the general population may have failed because they aimed at immunizing patients who already had anti-S. aureus immunity. Second,
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S. aureus differs from other pathogens in that it has evolved, as a commensal, several strategies to escape the immune system. These strategies include the destruction of phagocytic cells by several bacterial exotoxins, or the expression on the bacterial surface of the antibody- and opsonization-neutralizing protein A (Daum and Spellberg, 2012). Such strategies are likely to interfere with anti-S. aureus immunity, and should have been taken into account when designing vaccine candidates. Third, growing lines of evidence indicate that cell-mediated immunity could play an important role, if not prominent, in the protective immunity against S. aureus (Proctor, 2012), as illustrated by the fact that patients with AIDS are at higher risk of staphylococcal infection (Mathews et al., 2005; Wiese et al., 2013). Overall, vaccine candidates may have failed so far because they used too narrow sets of antigens to target a too versatile pathogen, armed with a wide range of virulence strategies and causing a wide range of diseases. One of the lessons to be learned from previous vaccine failures could be that the narrowing of vaccine indications is desirable. Rather than aiming at inducing a perhaps unachievable long-term protection in the general population against a widely disseminated and genetically diverse commensal, vaccine strategies could aim at obtaining a shorter-term protection in targeted populations such as patients undergoing surgery (Lee et al., 2010). However, protein antigens used in most S. aureus vaccine candidates take weeks to build up a protective T-cell-dependent humoral immunity, a delay that is hardly compatible with an effective prophylaxis against nosocomial S. aureus infection, especially in surgery and intensive care patients whose hospitalization is consecutive to a sudden disease or injury. To cope with this time constraint, S. aureus antigens could be arranged to specifically target the T-cell-independent arm of humoral immunity, which is able to induce a quick rise in antibody titers after a single immunization (Defrance et al., 2011), especially when the antigen is conjugated with a Toll-like receptor ligand (Schreiber, 2012). Alternative strategies could aim at stimulating the anti-S. aureus cell-mediated immunity along with the humoral immunity, in a similar fashion to what has been recently demonstrated with an experimental Streptococcus pneumoniae vaccine (Malley and Anderson, 2012).
6. Concluding remarks The famous futurist John Naisbitt once said, ‘‘we are drowning in information, but starving for knowledge’’ (Naisbett, 2002), an increasingly relevant situation with respect to the field of S. aureus research. The path from basic science achievements to measurable clinical successes against this pathogen has been tortuous and uneven, and will probably continue to be so. Nevertheless, recent encouraging evolutions such as the decline of MRSA in the UK prompt us to be optimistic rather than frustrated (Wyllie et al., 2011). S. aureus as an opportunistic pathogen is clearly not expected to vanish from the infectious diseases landscape, especially when one considers that advances in medical care and population ageing also result in a rise of the number of patients with high risk of S. aureus infection. Modern day research is increasingly endowed with technologies such as genomics or transcriptomics that yield considerable amounts of data (Beaume et al., 2010; Harris et al., 2010). Such data will translate to the clinical field only at the price of renewed efforts to identify and focus on the most relevant clinical questions. Promising results and successes of the recent years have probably arisen more frequently from the ceaseless confrontation of clinical, epidemiological and experimental findings, rather than from the sole improvement of the tools available to the researcher. We thus believe that multi-disciplinarity, combined with a bed-to-bench/
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
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bench-to-bed approach, is the best recipe to win the battle against staphylococcal diseases.
References Batra, R., Cooper, B.S., Whiteley, C., Patel, A.K., Wyncoll, D., Edgeworth, J.D., 2010. Efficacy and limitation of a chlorhexidine-based decolonization strategy in preventing transmission of methicillin-resistant Staphylococcus aureus in an intensive care unit. Clin. Infect. Dis. 50, 210–217. Beaume, M., Hernandez, D., Francois, P., Schrenzel, J., 2010. New approaches for functional genomic studies in staphylococci. Int. J. Med. Microbiol. 300, 88–97. Bode, L.G., Kluytmans, J.A., Wertheim, H.F., Bogaers, D., Vandenbroucke-Grauls, C.M., Roosendaal, R., Troelstra, A., Box, A.T., Voss, A., Van Der Tweel, I., Van Belkum, A., Verbrugh, H.A., Vos, M.C., 2010. Preventing surgical-site infections in nasal carriers of Staphylococcus aureus. N. Engl. J. Med. 362, 9–17. Boucher, H.W., Corey, G.R., 2008. Epidemiology of methicillin-resistant Staphylococcus aureus. Clin. Infect. Dis. 46 (Suppl. 5), S344–349. Centers for Disease Control and Prevention, 2005. Cases of HIV infection and AIDS in the United States, 2004. HIV/AIDS surveillance report vol. 16. Available at: (accessed 22.12.12). Chambers, H.F., 2005. Community-associated MRSA–resistance and virulence converge. N. Engl. J. Med. 352, 1485–1487. Corvaglia, A.R., Francois, P., Hernandez, D., Perron, K., Linder, P., Schrenzel, J., 2010. A type III-like restriction endonuclease functions as a major barrier to horizontal gene transfer in clinical Staphylococcus aureus strains. Proc. Natl. Acad. Sci. USA 107, 11954–11958. Daum, R.S., Spellberg, B., 2012. Progress toward a Staphylococcus aureus vaccine. Clin. Infect. Dis. 54, 560–567. Day, N.P., Moore, C.E., Enright, M.C., Berendt, A.R., Smith, J.M., Murphy, M.F., Peacock, S.J., Spratt, B.G., Feil, E.J., 2001. A link between virulence and ecological abundance in natural populations of Staphylococcus aureus. Science 292, 114– 116. Day, N.P., Moore, C.E., Enright, M.C., Berendt, A.P., Smith, J.M., Murphy, M.F., Peacock, S.J., Spratt, B.G., Feil, E.J., 2002. Retraction. Science 295, 971. Defrance, T., Taillardet, M., Genestier, L., 2011. T cell-independent B cell memory. Curr. Opin. Immunol. 23, 330–336. Deurenberg, R.H., Vink, C., Kalenic, S., Friedrich, A.W., Bruggeman, C.A., Stobberingh, E.E., 2007. The molecular evolution of methicillin-resistant Staphylococcus aureus. Clin. Microbiol. Infect. 13, 222–235. Diep, B.A., Carleton, H.A., Chang, R.F., Sensabaugh, G.F., Perdreau-Remington, F., 2006. Roles of 34 virulence genes in the evolution of hospital- and communityassociated strains of methicillin-resistant Staphylococcus aureus. J. Infect. Dis. 193, 1495–1503. Diep, B.A., Chan, L., Tattevin, P., Kajikawa, O., Martin, T.R., Basuino, L., Mai, T.T., Marbach, H., Braughton, K.R., Whitney, A.R., Gardner, D.J., Fan, X., Tseng, C.W., Liu, G.Y., Badiou, C., Etienne, J., Lina, G., Matthay, M.A., Deleo, F.R., Chambers, H.F., 2010. Polymorphonuclear leukocytes mediate Staphylococcus aureus Panton-Valentine leukocidin-induced lung inflammation and injury. Proc. Natl. Acad. Sci. USA 107, 5587–5592. Dohin, B., Gillet, Y., Kohler, R., Lina, G., Vandenesch, F., Vanhems, P., Floret, D., Etienne, J., 2007. Pediatric bone and joint infections caused by Panton-Valentine leukocidin-positive Staphylococcus aureus. Pediatr. Infect. Dis. J. 26, 1042–1048. Edgeworth, J.D., 2011. Has decolonization played a central role in the decline in UK methicillin-resistant Staphylococcus aureus transmission? A focus on evidence from intensive care. J. Antimicrob. Chemother. 66 (Suppl. 2), ii41–ii47. Faden, H., Lesse, A.J., Trask, J., Hill, J.A., Hess, D.J., Dryja, D., Lee, Y.H., 2010. Importance of colonization site in the current epidemic of staphylococcal skin abscesses. Pediatrics 125, e618–624. Gillet, Y., Issartel, B., Vanhems, P., Fournet, J.C., Lina, G., Bes, M., Vandenesch, F., Piemont, Y., Brousse, N., Floret, D., Etienne, J., 2002. Association between Staphylococcus aureus strains carrying gene for Panton-Valentine leukocidin and highly lethal necrotising pneumonia in young immunocompetent patients. Lancet 359, 753–759. Gillet, Y., Dumitrescu, O., Tristan, A., Dauwalder, O., Javouhey, E., Floret, D., Vandenesch, F., Etienne, J., Lina, G., 2011. Pragmatic management of PantonValentine leukocidin-associated staphylococcal diseases. Int. J. Antimicrob. Agents 38, 457–464. Gonzalez, B.E., Rueda, A.M., Shelburne 3rd, S.A., Musher, D.M., Hamill, R.J., Hulten, K.G., 2006. Community-associated strains of methicillin-resistant Staphylococccus aureus as the cause of healthcare-associated infection. Infect. Control. Hosp. Epidemiol. 27, 1051–1056. Gupta, S., Ferguson, N., Anderson, R., 1998. Chaos, persistence, and evolution of strain structure in antigenically diverse infectious agents. Science 280, 912– 915. Harris, S.R., Feil, E.J., Holden, M.T., Quail, M.A., Nickerson, E.K., Chantratita, N., Gardete, S., Tavares, A., Day, N., Lindsay, J.A., Edgeworth, J.D., De Lencastre, H., Parkhill, J., Peacock, S.J., Bentley, S.D., 2010. Evolution of MRSA during hospital transmission and intercontinental spread. Science 327, 469–474. Iwase, T., Uehara, Y., Shinji, H., Tajima, A., Seo, H., Takada, K., Agata, T., Mizunoe, Y., 2010. Staphylococcus epidermidis Esp inhibits Staphylococcus aureus biofilm formation and nasal colonization. Nature 465, 346–349. Kaito, C., Omae, Y., Matsumoto, Y., Nagata, M., Yamaguchi, H., Aoto, T., Ito, T., Hiramatsu, K., Sekimizu, K., 2008. A novel gene, fudoh, in the SCCmec region
suppresses the colony spreading ability and virulence of Staphylococcus aureus. PLoS ONE 3, e3921. Kaito, C., Saito, Y., Nagano, G., Ikuo, M., Omae, Y., Hanada, Y., Han, X., Kuwahara-Arai, K., Hishinuma, T., Baba, T., Ito, T., Hiramatsu, K., Sekimizu, K., 2011. Transcription and translation products of the cytolysin gene psm-mec on the mobile genetic element SCCmec regulate Staphylococcus aureus virulence. PLoS Pathog. 7, e1001267. Klevens, R.M., Morrison, M.A., Nadle, J., Petit, S., Gershman, K., Ray, S., Harrison, L.H., Lynfield, R., Dumyati, G., Townes, J.M., Craig, A.S., Zell, E.R., Fosheim, G.E., Mcdougal, L.K., Carey, R.B., Fridkin, S.K., 2007. Invasive methicillinresistant Staphylococcus aureus infections in the United States. JAMA 298, 1763–1771. Kluytmans, J.A., Mouton, J.W., Ijzerman, E.P., Vandenbroucke-Grauls, C.M., Maat, A.W., Wagenvoort, J.H., Verbrugh, H.A., 1995. Nasal carriage of Staphylococcus aureus as a major risk factor for wound infections after cardiac surgery. J. Infect. Dis. 171, 216–219. Knight, G.M., Budd, E.L., Whitney, L., Thornley, A., Al-Ghusein, H., Planche, T., Lindsay, J.A., 2012. Shift in dominant hospital-associated methicillin-resistant Staphylococcus aureus (HA-MRSA) clones over time. J. Antimicrob. Chemother. 67, 2514–2522. Kobayashi, S.D., Malachowa, N., Whitney, A.R., Braughton, K.R., Gardner, D.J., Long, D., Bubeck Wardenburg, J., Schneewind, O., Otto, M., Deleo, F.R., 2011. Comparative analysis of USA300 virulence determinants in a rabbit model of skin and soft tissue infection. J. Infect. Dis. 204, 937–941. Kreiswirth, B., Kornblum, J., Arbeit, R.D., Eisner, W., Maslow, J.N., Mcgeer, A., Low, D.E., Novick, R.P., 1993. Evidence for a clonal origin of methicillin resistance in Staphylococcus aureus. Science 259, 227–230. Labandeira-Rey, M., Couzon, F., Boisset, S., Brown, E.L., Bes, M., Benito, Y., Barbu, E.M., Vazquez, V., Hook, M., Etienne, J., Vandenesch, F., Bowden, M.G., 2007. Staphylococcus aureus Panton-Valentine leukocidin causes necrotizing pneumonia. Science 315, 1130–1133. Laurent, F., Lelievre, H., Cornu, M., Vandenesch, F., Carret, G., Etienne, J., Flandrois, J.P., 2001. Fitness and competitive growth advantage of new gentamicinsusceptible MRSA clones spreading in French hospitals. J. Antimicrob. Chemother. 47, 277–283. Lee, B.Y., Wiringa, A.E., Bailey, R.R., Lewis, G.J., Feura, J., Muder, R.R., 2010. Staphylococcus aureus vaccine for orthopedic patients: an economic model and analysis. Vaccine 28, 2465–2471. Lelievre, H., Lina, G., Jones, M.E., Olive, C., Forey, F., Roussel-Delvallez, M., NicolasChanoine, M.H., Bebear, C.M., Jarlier, V., Andremont, A., Vandenesch, F., Etienne, J., 1999. Emergence and spread in French hospitals of methicillin-resistant Staphylococcus aureus with increasing susceptibility to gentamicin and other antibiotics. J. Clin. Microbiol. 37, 3452–3457. Li, M., Cheung, G.Y., Hu, J., Wang, D., Joo, H.S., Deleo, F.R., Otto, M., 2010. Comparative analysis of virulence and toxin expression of global communityassociated methicillin-resistant Staphylococcus aureus strains. J. Infect. Dis. 202, 1866–1876. Lowy, F.D., 1998. Staphylococcus aureus infections. N. Engl. J. Med. 339, 520–532. Lowy, F.D., 2003. Antimicrobial resistance: the example of Staphylococcus aureus. J. Clin. Invest. 111, 1265–1273. Lozano, C., Gomez-Sanz, E., Benito, D., Aspiroz, C., Zarazaga, M., Torres, C., 2011. Staphylococcus aureus nasal carriage, virulence traits, antibiotic resistance mechanisms, and genetic lineages in healthy humans in Spain, with detection of CC398 and CC97 strains. Int. J. Med. Microbiol. 301, 500–505. Luzar, M.A., Coles, G.A., Faller, B., Slingeneyer, A., Dah, G.D., Briat, C., Wone, C., Knefati, Y., Kessler, M., Peluso, F., 1990. Staphylococcus aureus nasal carriage and infection in patients on continuous ambulatory peritoneal dialysis. N. Engl. J. Med. 322, 505–509. Maiden, M.C., Bygraves, J.A., Feil, E., Morelli, G., Russell, J.E., Urwin, R., Zhang, Q., Zhou, J., Zurth, K., Caugant, D.A., Feavers, I.M., Achtman, M., Spratt, B.G., 1998. Multilocus sequence typing: a portable approach to the identification of clones within populations of pathogenic microorganisms. Proc. Natl. Acad. Sci. USA 95, 3140–3145. Malley, R., Anderson, P.W., 2012. Serotype-independent pneumococcal experimental vaccines that induce cellular as well as humoral immunity. Proc. Natl. Acad. Sci. USA 109, 3623–3627. Maree, C.L., Daum, R.S., Boyle-Vavra, S., Matayoshi, K., Miller, L.G., 2007. Community-associated methicillin-resistant Staphylococcus aureus isolates causing healthcare-associated infections. Emerg. Infect. Dis. 13, 236–242. Mathews, W.C., Caperna, J.C., Barber, R.E., Torriani, F.J., Miller, L.G., May, S., Mccutchan, J.A., 2005. Incidence of and risk factors for clinically significant methicillin-resistant Staphylococcus aureus infection in a cohort of HIV-infected adults. J. Acquir. Immune Defic. Syndr. 40, 155–160. Mccormick, J.K., Yarwood, J.M., Schlievert, P.M., 2001. Toxic shock syndrome and bacterial superantigens: an update. Annu. Rev. Microbiol. 55, 77–104. Mcdonald, M., Dougall, A., Holt, D., Huygens, F., Oppedisano, F., Giffard, P.M., InmanBamber, J., Stephens, A.J., Towers, R., Carapetis, J.R., Currie, B.J., 2006. Use of a single-nucleotide polymorphism genotyping system to demonstrate the unique epidemiology of methicillin-resistant Staphylococcus aureus in remote aboriginal communities. J. Clin. Microbiol. 44, 3720–3727. Mcdougal, L.K., Steward, C.D., Killgore, G.E., Chaitram, J.M., Mcallister, S.K., Tenover, F.C., 2003. Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: establishing a national database. J. Clin. Microbiol. 41, 5113–5120. Melles, D.C., Gorkink, R.F., Boelens, H.A., Snijders, S.V., Peeters, J.K., Moorhouse, M.J., Van Der Spek, P.J., Van Leeuwen, W.B., Simons, G., Verbrugh, H.A., Van Belkum,
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
J.-P. Rasigade, F. Vandenesch / Infection, Genetics and Evolution xxx (2013) xxx–xxx A., 2011. Natural population dynamics and expansion of pathogenic clones of Staphylococcus aureus. J. Clin. Invest. 114, 1732–1740. Montgomery, C.P., Boyle-Vavra, S., Adem, P.V., Lee, J.C., Husain, A.N., Clasen, J., Daum, R.S., 2008. Comparison of virulence in community-associated methicillin-resistant Staphylococcus aureus pulsotypes USA300 and USA400 in a rat model of pneumonia. J. Infect. Dis. 198, 561–570. Montgomery, C.P., Boyle-Vavra, S., Daum, R.S., 2010. Importance of the global regulators Agr and SaeRS in the pathogenesis of CA-MRSA USA300 infection. PLoS One 5, e15177. Naisbett, J., 2002. High tech. High touch. John Naisbett’s world view. Caring 21, 24–27. Nishifuji, K., Sugai, M., Amagai, M., 2008. Staphylococcal exfoliative toxins: ‘‘molecular scissors’’ of bacteria that attack the cutaneous defense barrier in mammals. J. Dermatol. Sci. 49, 21–31. Nubel, U., Roumagnac, P., Feldkamp, M., Song, J.H., Ko, K.S., Huang, Y.C., Coombs, G., Ip, M., Westh, H., Skov, R., Struelens, M.J., Goering, R.V., Strommenger, B., Weller, A., Witte, W., Achtman, M., 2008. Frequent emergence and limited geographic dispersal of methicillin-resistant Staphylococcus aureus. Proc. Natl. Acad. Sci. USA 105, 14130–14135. Otto, M., 2010. Basis of virulence in community-associated methicillin-resistant Staphylococcus aureus. Annu. Rev. Microbiol. 64, 143–162. Patel, M., Hoesley, C.J., Moser, S.A., Stamm, A.M., Baddley, J.W., Waites, K.B., 2008. Dissemination of community-associated methicillin-resistant Staphylococcus aureus in a tertiary care hospital. South Med. J. 101, 40–45. Piechowicz, L., Garbacz, K., Wisniewska, K., Dabrowska-Szponar, M., 2011. Screening of Staphylococcus aureus nasal strains isolated from medical students for toxin genes. Folia Microbiol. (Praha) 56, 225–229. Proctor, R.A., 2012. Challenges for a universal Staphylococcus aureus vaccine. Clin. Infect. Dis. 54, 1179–1186. Rasigade, J.P., Laurent, F., Lina, G., Meugnier, H., Bes, M., Vandenesch, F., Etienne, J., Tristan, A., 2010. Global distribution and evolution of Panton-Valentine leukocidin-positive methicillin-susceptible Staphylococcus aureus, 1981– 2007. J. Infect. Dis. 201, 1589–1597. Rouzic, N., Janvier, F., Libert, N., Javouhey, E., Lina, G., Nizou, J.Y., Pasquier, P., Stamm, D., Brinquin, L., Pelletier, C., Vandenesch, F., Floret, D., Etienne, J., Gillet, Y., 2010. Prompt and successful toxin-targeting treatment of three patients with necrotizing pneumonia due to Staphylococcus aureus strains carrying the Panton-Valentine leukocidin genes. J. Clin. Microbiol. 48, 1952–1955. Schijffelen, M.J., Boel, C.H., Van Strijp, J.A., Fluit, A.C., 2010. Whole genome analysis of a livestock-associated methicillin-resistant Staphylococcus aureus ST398 isolate from a case of human endocarditis. BMC Genomics 11, 376. Schreiber, J.R., 2012. Role of Toll like receptors in the antibody response to encapsulated bacteria. Front Biosci. (Elite Ed) 4, 2638–2646. Seale, A., Finn, A., 2011. What is the best way to use conjugate vaccines? Curr. Opin. Infect. Dis. 24, 219–224. Sivaraman, K., Venkataraman, N., Cole, A.M., 2009. Staphylococcus aureus nasal carriage and its contributing factors. Future Microbiol. 4, 999–1008.
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Tacconelli, E., Carmeli, Y., Aizer, A., Ferreira, G., Foreman, M.G., D’agata, E.M., 2003. Mupirocin prophylaxis to prevent Staphylococcus aureus infection in patients undergoing dialysis: a meta-analysis. Clin. Infect. Dis. 37, 1629–1638. Tristan, A., Bes, M., Meugnier, H., Lina, G., Bozdogan, B., Courvalin, P., Reverdy, M.E., Enright, M.C., Vandenesch, F., Etienne, J., 2007. Global distribution of PantonValentine leukocidin–positive methicillin-resistant Staphylococcus aureus, 2006. Emerg. Infect. Dis. 13, 594–600. Vandenesch, F., Naimi, T., Enright, M.C., Lina, G., Nimmo, G.R., Heffernan, H., Liassine, N., Bes, M., Greenland, T., Reverdy, M.E., Etienne, J., 2003. Community-acquired methicillin-resistant Staphylococcus aureus carrying Panton-Valentine leukocidin genes: worldwide emergence. Emerg. Infect. Dis. 9, 978–984. Verkaik, N.J., Lebon, A., De Vogel, C.P., Hooijkaas, H., Verbrugh, H.A., Jaddoe, V.W., Hofman, A., Moll, H.A., Van Belkum, A., Van Wamel, W.J., 2010. Induction of antibodies by Staphylococcus aureus nasal colonization in young children. Clin. Microbiol. Infect. 16, 1312–1317. Von Eiff, C., Becker, K., Machka, K., Stammer, H., Peters, G., 2001. Nasal carriage as a source of Staphylococcus aureus bacteremia. Study group. N. Engl. J. Med. 344, 11–16. Wang, R., Braughton, K.R., Kretschmer, D., Bach, T.H., Queck, S.Y., Li, M., Kennedy, A.D., Dorward, D.W., Klebanoff, S.J., Peschel, A., Deleo, F.R., Otto, M., 2007. Identification of novel cytolytic peptides as key virulence determinants for community-associated MRSA. Nat. Med. 13, 1510–1514. Wertheim, H.F., Vos, M.C., Ott, A., Van Belkum, A., Voss, A., Kluytmans, J.A., Van Keulen, P.H., Vandenbroucke-Grauls, C.M., Meester, M.H., Verbrugh, H.A., 2004. Risk and outcome of nosocomial Staphylococcus aureus bacteraemia in nasal carriers versus non-carriers. Lancet 364, 703–705. Wiese, L., Mejer, N., Schonheyder, H.C., Westh, H., Jensen, A.G., Larsen, A.R., Skov, R., Benfield, T., 2013. A nationwide study of comorbidity and risk of reinfection after Staphylococcus aureus bacteraemia. J. Infect. 67, 199–205. Wyllie, D., Paul, J., Crook, D., 2011. Waves of trouble: MRSA strain dynamics and assessment of the impact of infection control. J. Antimicrob. Chemother. 66, 2685–2688. Yanagisawa, C., Hanaki, H., Natae, T., Sunakawa, K., 2007. Neutralization of staphylococcal exotoxins in vitro by human-origin intravenous immunoglobulin. J. Infect. Chemother. 13, 368–372. Yang, E.S., Tan, J., Eells, S., Rieg, G., Tagudar, G., Miller, L.G., 2010. Body site colonization in patients with community-associated methicillin-resistant Staphylococcus aureus and other types of S. aureus skin infections. Clin. Microbiol. Infect. 16, 425–431. Zielinska, A.K., Beenken, K.E., Joo, H.S., Mrak, L.N., Griffin, L.M., Luong, T.T., Lee, C.Y., Otto, M., Shaw, L.N., Smeltzer, M.S., 2011. Defining the strain-dependent impact of the Staphylococcal accessory regulator (sarA) on the alpha-toxin phenotype of Staphylococcus aureus. J. Bacteriol. 193, 2948–2958.
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Staphylococcus aureus: A pathogen with still unresolved issues Jean-Philippe Rasigade, François Vandenesch ⇑ UMR U1111 INSERM Université de Lyon, Site Laënnec, 8 rue Guillaume Paradin, 69372 Lyon, France Hospices Civils de Lyon, Groupement Hospitalier Est, 69 Boulevard Pinel, 69677 Bron, France
a r t i c l e
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Article history: Received 1 August 2013 Received in revised form 21 August 2013 Accepted 22 August 2013 Available online xxxx Keywords: Staphylococcus aureus MRSA Molecular epidemiology Bacterial toxins Gene expression regulation Vaccine strategy
a b s t r a c t Staphylococcus aureus is a major human pathogen, and considerable research efforts have been put forward to improve our understanding of its complex pathogenesis. In spite of these efforts, the burden of staphylococcal infections is still on the rise. This review focuses on a selected set of crucial unresolved questions regarding this pathogen, namely: (i) the nature of the driving forces behind the rise and decline of methicillin-resistant S. aureus (MRSA) clones; (ii) the mechanisms by which a commensal becomes a pathogen; (iii) the molecular underpinnings of toxin overexpression in hypervirulent MRSA clones such as USA300; and (iv) the repeated failures of anti-S. aureus vaccine approaches. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Staphylococcus aureus is one of the most important human pathogens both in the hospital and the community (Lowy, 1998). Since the beginning of the antibiotics era, S. aureus has been a privileged target of therapeutics research, and numerous antimicrobial agents, as well as vaccine attempts, were specifically designed to combat this bacterium (Lowy, 2003; Proctor, 2012). The overall amount of research efforts aimed at understanding S. aureus is still on the rise: searching the PubMed database with the ‘‘S. aureus’’ keywords over the 2007–2012 period retrieved 21060 articles, a 51.4% increase as compared to the 13907 articles published during the 2002–2007 period. Such efforts in both the fields of basic science and clinical research could have been expected, from a clinical standpoint, to result in dramatic improvements in cure rates of S. aureus-infected patients as well as in a global decrease of the incidence of S. aureus infections. However, the history of the last decades clearly shows that these expectations were not met. In 2005, methicillin-resistant S. aureus infection has become a more common cause of death in US patients than HIV infection (Centers for Disease Control and Prevention, 2005; Klevens et al., 2007). Besides this striking US-specific figure lies a more global threat: the increase of MRSA infection incidence in several regions of the world, not only in the hospital but also in the community (Boucher and Corey, 2008). Although our understanding of the lifestyle and the pathogenic strategies of S. aureus is constantly increasing, several important questions have remained unresolved. This review will ⇑ Corresponding author. Tel.: +33 472357252. E-mail address: [email protected] (F. Vandenesch).
focus on these questions, which could be major obstacles against our efforts to translate the scientific achievements on S. aureus into clinical successes.
2. The rise and decline of MRSA clones MRSA is resistant to all currently available beta-lactam antibiotics. Although this pathogen emerged rapidly after the introduction of methicillin in clinical practice in the early 1960s, it only became a public health threat during the 1980s. Its expansion was accompanied by the acquisition of resistance determinants to non-betalactam antibiotics, which led some authors to suggest that betalactam resistance alone was not sufficient to promote MRSA dissemination in the hospital (Knight et al., 2012). The spread of MRSA worldwide was due to a limited number of successful clones as determined using molecular typing techniques such as pulsedfield gel electrophoresis (PFGE) or multi-locus sequence typing (MLST) (Maiden et al., 1998; Mcdougal et al., 2003). MRSA is polyphyletic, meaning that the MRSA clones responsible for the past and current epidemics do not share a common MRSA ancestor. Indeed, MRSA emerged from methicillin-susceptible S. aureus (MSSA) through the horizontal transfer of a mobile genetic element, the staphylococcal cassette chromosome mec (SCCmec). Contrasting with previous studies favoring a clonal origin of methicillin resistance in S. aureus (Kreiswirth et al., 1993), more recent phylogenetic studies indicated that SCCmec acquisition was a frequent event in S. aureus evolution (Nubel et al., 2008), with MRSA subgroups such as ST36 or ST80 originating from genetically distinct MSSA lineages (Deurenberg et al., 2007; Rasigade et al., 2010). A
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Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
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consequence of this polyphyletic nature of MRSA is that it can hardly be studied as a whole because different MRSA lineages often retain the different genotypic and phenotypic traits of their respective MSSA ancestors, and acquire additional features that reflect their specific evolutionary history. As a consequence, MRSA clones show striking differences in terms of fitness, virulence and the spectrum of resistance (Diep et al., 2006; Li et al., 2010). MRSA emergence and spread in the hospital and more recently in the community, is the result of consecutive and often intermingled epidemic waves due to different clones. Early MRSA strains belonged to the ST250 lineage, which is part of the highly successful clonal complex (CC) 8. This Archaic clone virtually disappeared from hospital in the late 1970s and was gradually replaced by other MRSA lineages. Some of these new lineages were genetically related to the Archaic clone but harbored different allotypes of the SCCmec resistance determinant, such as the ST247 Iberian clone or the ST239 EMRSA-1 clone, while some other lineages were genetically unrelated to CC8, such as the ST5 USA100 clone or the ST36 EMRSA-16 clone. The molecular epidemiology of MRSA has remained extremely dynamic, and dramatic changes in the relative abundance of each clone have occurred over the last two decades with, for instance, the replacement of the ST36 EMRSA-16 clone by the ST22 EMRSA-15 clone as the major cause of MRSA infection in UK hospitals (Wyllie et al., 2011; Knight et al., 2012), or the emergence of highly epidemic clones of community-acquired (CA-) MRSA such as ST8 USA300 in the US, which is currently replacing the ST5 USA100 clone in the hospital setting (Gonzalez et al., 2006; Maree et al., 2007; Patel et al., 2008). A thorough understanding of the driving forces that rule the expansion and decline of these different clones is clearly needed to help improve infection control strategies and contain the MRSA epidemics. However, our knowledge of these driving forces is still limited. The most obvious contributing factor to MRSA dynamics is antibiotics use and overuse. Nevertheless, several other important factors likely contribute to MRSA epidemics. For instance, high rates of MRSA have been recorded from remote populations with limited access to the healthcare system or antibiotics prescription (Mcdonald et al., 2006), thus indicating that antibiotics selection pressure is not sufficient to explain MRSA emergence. Similarly, infection control enforcement do not always correlate with the decline of a given MRSA clone: in the UK, the decline of the ST36 EMRSA-16 lineage appeared to have begun before that new infection control policies took place (Wyllie et al., 2011). During the 1990s, the decline in France of the gentamicin-resistant MRSA clone, which was replaced by the gentamicin-susceptible MRSA Lyon clone, was not explained by a reduction of gentamicin consumption (Lelievre et al., 1999), but maybe by a differential fitness of the two lineages (Laurent et al., 2001). Indeed, several clone-specific features could be linked with the emergence or decline of a peculiar lineage. Bacterial competition could occur between clones that share the same niche, similar to what has been demonstrated for S. aureus and S. epidermidis (Iwase et al., 2010). As mentioned above (Laurent et al., 2001), differences in bacterial fitness such as those observed between the highly successful ST22 EMRSA-15 clone and the currently declining CC30 MRSA are also potentially involved (Knight et al., 2012). Alternatively, the acquisition of immunity in the community in contact with a given clone could play a role in the subsequent decline of this clone (Gupta et al., 1998). Finally, lineage-specific features such as chlorhexidine resistance have been shown to influence MRSA dynamics in settings where infection control guidelines encourage the use of this antiseptic (Batra et al., 2010; Edgeworth, 2011). Overall, MRSA dynamics appear to be ruled by a complex interplay between human interventions, bacterial evolution and clone-specific biological behaviors, the consequence of which being our current inability to predict the epidemic potential of newly emerging clones.
3. What turns commensal strains into invasive strains? S. aureus is carried without damage in 30% of humans, mostly in the nose but also in the throat, gut and skin folds (Sivaraman et al., 2009; Faden et al., 2010; Yang et al., 2010). Carriage increases the risk of developing a staphylococcal infection, and healthcare-associated infections are usually caused by the strains carried in the patient nose (Luzar et al., 1990; Kluytmans et al., 1995; Von Eiff et al., 2001; Wertheim et al., 2004). An important yet still unanswered question is whether genotypic differences exist between harmless carriage isolates and those that cause invasive infections. Several molecular typing studies using either MLST or amplified fragment length polymorphism failed to demonstrate such a difference (Day et al., 2001, 2002; Melles et al., 2004), and there is no evidence that lineages exist that are specifically associated with either colonization or disease. Indeed, although the presence of specific genes has been undoubtedly associated with specific clinical entities, such as the exfoliative toxins with staphylococcal scalded skin syndrome, or superantigens such as TSST-1 with toxic shock syndromes, isolates harboring such toxins are also found in healthy carriers (Mccormick et al., 2001; Nishifuji et al., 2008; Lozano et al., 2011; Piechowicz et al., 2011). Another unanswered question is whether S. aureus colonization is beneficial or detrimental to the carrier. On the one hand, as stated above, colonized sites can act as a reservoir for healthcareassociated infections in compromised patients. Carriage is thus righteously considered detrimental in patients at risk for staphylococcal nosocomial infection, and decolonization measures have proven efficient at reducing the risk of staphylococcal infection in colonized patients undergoing hemodialysis or surgery (Tacconelli et al., 2003; Bode et al., 2010). On the other hand, S. aureus carriers that develop S. aureus bacteremia have a lower risk of death than non-carriers (Wertheim et al., 2004), suggesting that S. aureus colonization can induce some form of immunological adaptation beneficial for the host in terms of outcome if an invasive infection occurs.
4. Toxin expression regulation in S. aureus The molecular basis of virulence in S. aureus has been an active field of research for several decades, but such research has further intensified and become decisive with the recent emergence of highly pathogenic CA-MRSA strains that combined antibiotics resistance, rapid spreading ability and exceptional virulence (Chambers, 2005). The last decade has seen the identification of yet unrecognized S. aureus virulence factors such as the phenol-soluble modulins (Wang et al., 2007), as well as the characterization of the pathogenic role of long-known toxins such as the PantonValentine leukocidin (Gillet et al., 2002; Labandeira-Rey et al., 2007; Diep et al., 2010). Vast amounts of work have been necessary to gain understanding of the complex multifactorial nature of CAMRSA virulence and to outline which genetic features make one S. aureus strain more virulent than another. A clear distinction has been made between, on the one hand, virulence gained through the acquisition of new toxin genes by horizontal transfer such as the phage-borne pvl genes, and on the other hand, virulence gained through the overexpression of core genome-encoded toxins such as PSMs or alpha-toxin (Wang et al., 2007; Li et al., 2010; Otto, 2010). An additional layer of complexity has also added to this already intricate scheme with the observation that the acquisition of new genes harbored by mobile genetic elements (MGEs) correlated with modifications in the expression level of core genome-encoded toxin genes (Kaito et al., 2008, 2011). Gaining a deeper understanding of the molecular pathogenic mechanisms of CA-MRSA is crucial as a prerequisite to the design of dedicated therapeutic strategies,
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
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including those aimed at targeting specific toxins such as PVL or superantigens (Yanagisawa et al., 2007; Rouzic et al., 2010; Gillet et al., 2011). Moreover, lessons learned from CA-MRSA virulence research could help understand virulence in MSSA, which are still the main cause of severe community-acquired staphylococcal disease in Europe (Dohin et al., 2007; Gillet et al., 2011). In spite of these research efforts, several important questions remain open with respect to CA-MRSA virulence and, more generally, S. aureus lineage-specific virulence. First, the genetic features that lie behind the ability of the strains of a given lineage to stably integrate a given MGE are largely unknown. There is strong evidence that MGEs such as SCCmec cassettes or the PVL-encoding phages only integrate in a subset of S. aureus lineages (Vandenesch et al., 2003; Tristan et al., 2007), and both these MGEs seem to preferentially integrate into the same lineages (Rasigade et al., 2010). Lineage-specific differences in the defenses against foreign DNA such as restriction-modification systems (Corvaglia et al., 2010; Schijffelen et al., 2010) could tentatively account for the observed associations of specific MGEs with specific lineages, but this hypothesis warrants further research. The second question is that of the genetic features that account for lineage-specific differences in toxin gene expression levels. Recent reports have underlined the decisive role of the major regulators agr, sarA and saeRS in the increased virulence of CA-MRSA strains (Montgomery et al., 2008, 2010; Li et al., 2010; Kobayashi et al., 2011; Zielinska et al., 2011). Such regulators are expressed at higher levels in CA-MRSA strains such as USA300 as compared to other S. aureus lineages (Montgomery et al., 2008, 2010). Regulator overexpression, by upregulating toxin transcription and secretion, provides a comprehensive explanation to CA-MRSA virulence along with the presence of specific toxin genes such as pvl (Otto, 2010). However, the mechanisms by which such regulators are overexpressed in some specific S. aureus lineages are still unknown, and the discovery of these mechanisms will probably be one of the major challenges of the ‘‘omics’’ era.
5. The repeated failures of S.aureus vaccines One the most decisive advances in our struggle against bacterial diseases results from the development of vaccines effective at preventing serious conditions due to, e.g., Neisseria meningitidis, Haemophilus influenzae type b or Streptococcus pneumoniae (Seale and Finn, 2011). All these vaccines were aimed at eliciting protective immunization through a specific antibody response. Several approaches to induce a protective immunity against S. aureus have been made (Proctor, 2012). Understandably, all these vaccine candidates comprised selected antigens targeting humoral immunity; all showed promising results in pre-clinical animal models, but all these vaccine strategies failed in clinical trials (Daum and Spellberg, 2012). What went wrong? Understanding the differences between S. aureus and the bacteria against which successful vaccines were obtained is a crucial issue to help develop effective S. aureus vaccines in the future. First, pathogens for which vaccine attempts were successful had well-characterized pathogenic mechanisms, linked to a limited number of surface-exposed epitopes or secreted toxins. The multiplicity and frequent redundancy of S. aureus virulence factors, as well as the broad spectrum of diseases caused by this pathogen (Lowy, 1998), could help explain why the optimal antigen or combination of antigens has eluded us so far. S. aureus lives with humans as normal flora, the consequence of which being that virtually all humans possess anti-S. aureus antibodies (Verkaik et al., 2010). Hence, antibody-based vaccine attempts in the general population may have failed because they aimed at immunizing patients who already had anti-S. aureus immunity. Second,
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S. aureus differs from other pathogens in that it has evolved, as a commensal, several strategies to escape the immune system. These strategies include the destruction of phagocytic cells by several bacterial exotoxins, or the expression on the bacterial surface of the antibody- and opsonization-neutralizing protein A (Daum and Spellberg, 2012). Such strategies are likely to interfere with anti-S. aureus immunity, and should have been taken into account when designing vaccine candidates. Third, growing lines of evidence indicate that cell-mediated immunity could play an important role, if not prominent, in the protective immunity against S. aureus (Proctor, 2012), as illustrated by the fact that patients with AIDS are at higher risk of staphylococcal infection (Mathews et al., 2005; Wiese et al., 2013). Overall, vaccine candidates may have failed so far because they used too narrow sets of antigens to target a too versatile pathogen, armed with a wide range of virulence strategies and causing a wide range of diseases. One of the lessons to be learned from previous vaccine failures could be that the narrowing of vaccine indications is desirable. Rather than aiming at inducing a perhaps unachievable long-term protection in the general population against a widely disseminated and genetically diverse commensal, vaccine strategies could aim at obtaining a shorter-term protection in targeted populations such as patients undergoing surgery (Lee et al., 2010). However, protein antigens used in most S. aureus vaccine candidates take weeks to build up a protective T-cell-dependent humoral immunity, a delay that is hardly compatible with an effective prophylaxis against nosocomial S. aureus infection, especially in surgery and intensive care patients whose hospitalization is consecutive to a sudden disease or injury. To cope with this time constraint, S. aureus antigens could be arranged to specifically target the T-cell-independent arm of humoral immunity, which is able to induce a quick rise in antibody titers after a single immunization (Defrance et al., 2011), especially when the antigen is conjugated with a Toll-like receptor ligand (Schreiber, 2012). Alternative strategies could aim at stimulating the anti-S. aureus cell-mediated immunity along with the humoral immunity, in a similar fashion to what has been recently demonstrated with an experimental Streptococcus pneumoniae vaccine (Malley and Anderson, 2012).
6. Concluding remarks The famous futurist John Naisbitt once said, ‘‘we are drowning in information, but starving for knowledge’’ (Naisbett, 2002), an increasingly relevant situation with respect to the field of S. aureus research. The path from basic science achievements to measurable clinical successes against this pathogen has been tortuous and uneven, and will probably continue to be so. Nevertheless, recent encouraging evolutions such as the decline of MRSA in the UK prompt us to be optimistic rather than frustrated (Wyllie et al., 2011). S. aureus as an opportunistic pathogen is clearly not expected to vanish from the infectious diseases landscape, especially when one considers that advances in medical care and population ageing also result in a rise of the number of patients with high risk of S. aureus infection. Modern day research is increasingly endowed with technologies such as genomics or transcriptomics that yield considerable amounts of data (Beaume et al., 2010; Harris et al., 2010). Such data will translate to the clinical field only at the price of renewed efforts to identify and focus on the most relevant clinical questions. Promising results and successes of the recent years have probably arisen more frequently from the ceaseless confrontation of clinical, epidemiological and experimental findings, rather than from the sole improvement of the tools available to the researcher. We thus believe that multi-disciplinarity, combined with a bed-to-bench/
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
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bench-to-bed approach, is the best recipe to win the battle against staphylococcal diseases.
References Batra, R., Cooper, B.S., Whiteley, C., Patel, A.K., Wyncoll, D., Edgeworth, J.D., 2010. Efficacy and limitation of a chlorhexidine-based decolonization strategy in preventing transmission of methicillin-resistant Staphylococcus aureus in an intensive care unit. Clin. Infect. Dis. 50, 210–217. Beaume, M., Hernandez, D., Francois, P., Schrenzel, J., 2010. New approaches for functional genomic studies in staphylococci. Int. J. Med. Microbiol. 300, 88–97. Bode, L.G., Kluytmans, J.A., Wertheim, H.F., Bogaers, D., Vandenbroucke-Grauls, C.M., Roosendaal, R., Troelstra, A., Box, A.T., Voss, A., Van Der Tweel, I., Van Belkum, A., Verbrugh, H.A., Vos, M.C., 2010. Preventing surgical-site infections in nasal carriers of Staphylococcus aureus. N. Engl. J. Med. 362, 9–17. Boucher, H.W., Corey, G.R., 2008. Epidemiology of methicillin-resistant Staphylococcus aureus. Clin. Infect. Dis. 46 (Suppl. 5), S344–349. Centers for Disease Control and Prevention, 2005. Cases of HIV infection and AIDS in the United States, 2004. HIV/AIDS surveillance report vol. 16. Available at:
suppresses the colony spreading ability and virulence of Staphylococcus aureus. PLoS ONE 3, e3921. Kaito, C., Saito, Y., Nagano, G., Ikuo, M., Omae, Y., Hanada, Y., Han, X., Kuwahara-Arai, K., Hishinuma, T., Baba, T., Ito, T., Hiramatsu, K., Sekimizu, K., 2011. Transcription and translation products of the cytolysin gene psm-mec on the mobile genetic element SCCmec regulate Staphylococcus aureus virulence. PLoS Pathog. 7, e1001267. Klevens, R.M., Morrison, M.A., Nadle, J., Petit, S., Gershman, K., Ray, S., Harrison, L.H., Lynfield, R., Dumyati, G., Townes, J.M., Craig, A.S., Zell, E.R., Fosheim, G.E., Mcdougal, L.K., Carey, R.B., Fridkin, S.K., 2007. Invasive methicillinresistant Staphylococcus aureus infections in the United States. JAMA 298, 1763–1771. Kluytmans, J.A., Mouton, J.W., Ijzerman, E.P., Vandenbroucke-Grauls, C.M., Maat, A.W., Wagenvoort, J.H., Verbrugh, H.A., 1995. Nasal carriage of Staphylococcus aureus as a major risk factor for wound infections after cardiac surgery. J. Infect. Dis. 171, 216–219. Knight, G.M., Budd, E.L., Whitney, L., Thornley, A., Al-Ghusein, H., Planche, T., Lindsay, J.A., 2012. Shift in dominant hospital-associated methicillin-resistant Staphylococcus aureus (HA-MRSA) clones over time. J. Antimicrob. Chemother. 67, 2514–2522. Kobayashi, S.D., Malachowa, N., Whitney, A.R., Braughton, K.R., Gardner, D.J., Long, D., Bubeck Wardenburg, J., Schneewind, O., Otto, M., Deleo, F.R., 2011. Comparative analysis of USA300 virulence determinants in a rabbit model of skin and soft tissue infection. J. Infect. Dis. 204, 937–941. Kreiswirth, B., Kornblum, J., Arbeit, R.D., Eisner, W., Maslow, J.N., Mcgeer, A., Low, D.E., Novick, R.P., 1993. Evidence for a clonal origin of methicillin resistance in Staphylococcus aureus. Science 259, 227–230. Labandeira-Rey, M., Couzon, F., Boisset, S., Brown, E.L., Bes, M., Benito, Y., Barbu, E.M., Vazquez, V., Hook, M., Etienne, J., Vandenesch, F., Bowden, M.G., 2007. Staphylococcus aureus Panton-Valentine leukocidin causes necrotizing pneumonia. Science 315, 1130–1133. Laurent, F., Lelievre, H., Cornu, M., Vandenesch, F., Carret, G., Etienne, J., Flandrois, J.P., 2001. Fitness and competitive growth advantage of new gentamicinsusceptible MRSA clones spreading in French hospitals. J. Antimicrob. Chemother. 47, 277–283. Lee, B.Y., Wiringa, A.E., Bailey, R.R., Lewis, G.J., Feura, J., Muder, R.R., 2010. Staphylococcus aureus vaccine for orthopedic patients: an economic model and analysis. Vaccine 28, 2465–2471. Lelievre, H., Lina, G., Jones, M.E., Olive, C., Forey, F., Roussel-Delvallez, M., NicolasChanoine, M.H., Bebear, C.M., Jarlier, V., Andremont, A., Vandenesch, F., Etienne, J., 1999. Emergence and spread in French hospitals of methicillin-resistant Staphylococcus aureus with increasing susceptibility to gentamicin and other antibiotics. J. Clin. Microbiol. 37, 3452–3457. Li, M., Cheung, G.Y., Hu, J., Wang, D., Joo, H.S., Deleo, F.R., Otto, M., 2010. Comparative analysis of virulence and toxin expression of global communityassociated methicillin-resistant Staphylococcus aureus strains. J. Infect. Dis. 202, 1866–1876. Lowy, F.D., 1998. Staphylococcus aureus infections. N. Engl. J. Med. 339, 520–532. Lowy, F.D., 2003. Antimicrobial resistance: the example of Staphylococcus aureus. J. Clin. Invest. 111, 1265–1273. Lozano, C., Gomez-Sanz, E., Benito, D., Aspiroz, C., Zarazaga, M., Torres, C., 2011. Staphylococcus aureus nasal carriage, virulence traits, antibiotic resistance mechanisms, and genetic lineages in healthy humans in Spain, with detection of CC398 and CC97 strains. Int. J. Med. Microbiol. 301, 500–505. Luzar, M.A., Coles, G.A., Faller, B., Slingeneyer, A., Dah, G.D., Briat, C., Wone, C., Knefati, Y., Kessler, M., Peluso, F., 1990. Staphylococcus aureus nasal carriage and infection in patients on continuous ambulatory peritoneal dialysis. N. Engl. J. Med. 322, 505–509. Maiden, M.C., Bygraves, J.A., Feil, E., Morelli, G., Russell, J.E., Urwin, R., Zhang, Q., Zhou, J., Zurth, K., Caugant, D.A., Feavers, I.M., Achtman, M., Spratt, B.G., 1998. Multilocus sequence typing: a portable approach to the identification of clones within populations of pathogenic microorganisms. Proc. Natl. Acad. Sci. USA 95, 3140–3145. Malley, R., Anderson, P.W., 2012. Serotype-independent pneumococcal experimental vaccines that induce cellular as well as humoral immunity. Proc. Natl. Acad. Sci. USA 109, 3623–3627. Maree, C.L., Daum, R.S., Boyle-Vavra, S., Matayoshi, K., Miller, L.G., 2007. Community-associated methicillin-resistant Staphylococcus aureus isolates causing healthcare-associated infections. Emerg. Infect. Dis. 13, 236–242. Mathews, W.C., Caperna, J.C., Barber, R.E., Torriani, F.J., Miller, L.G., May, S., Mccutchan, J.A., 2005. Incidence of and risk factors for clinically significant methicillin-resistant Staphylococcus aureus infection in a cohort of HIV-infected adults. J. Acquir. Immune Defic. Syndr. 40, 155–160. Mccormick, J.K., Yarwood, J.M., Schlievert, P.M., 2001. Toxic shock syndrome and bacterial superantigens: an update. Annu. Rev. Microbiol. 55, 77–104. Mcdonald, M., Dougall, A., Holt, D., Huygens, F., Oppedisano, F., Giffard, P.M., InmanBamber, J., Stephens, A.J., Towers, R., Carapetis, J.R., Currie, B.J., 2006. Use of a single-nucleotide polymorphism genotyping system to demonstrate the unique epidemiology of methicillin-resistant Staphylococcus aureus in remote aboriginal communities. J. Clin. Microbiol. 44, 3720–3727. Mcdougal, L.K., Steward, C.D., Killgore, G.E., Chaitram, J.M., Mcallister, S.K., Tenover, F.C., 2003. Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: establishing a national database. J. Clin. Microbiol. 41, 5113–5120. Melles, D.C., Gorkink, R.F., Boelens, H.A., Snijders, S.V., Peeters, J.K., Moorhouse, M.J., Van Der Spek, P.J., Van Leeuwen, W.B., Simons, G., Verbrugh, H.A., Van Belkum,
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018
J.-P. Rasigade, F. Vandenesch / Infection, Genetics and Evolution xxx (2013) xxx–xxx A., 2011. Natural population dynamics and expansion of pathogenic clones of Staphylococcus aureus. J. Clin. Invest. 114, 1732–1740. Montgomery, C.P., Boyle-Vavra, S., Adem, P.V., Lee, J.C., Husain, A.N., Clasen, J., Daum, R.S., 2008. Comparison of virulence in community-associated methicillin-resistant Staphylococcus aureus pulsotypes USA300 and USA400 in a rat model of pneumonia. J. Infect. Dis. 198, 561–570. Montgomery, C.P., Boyle-Vavra, S., Daum, R.S., 2010. Importance of the global regulators Agr and SaeRS in the pathogenesis of CA-MRSA USA300 infection. PLoS One 5, e15177. Naisbett, J., 2002. High tech. High touch. John Naisbett’s world view. Caring 21, 24–27. Nishifuji, K., Sugai, M., Amagai, M., 2008. Staphylococcal exfoliative toxins: ‘‘molecular scissors’’ of bacteria that attack the cutaneous defense barrier in mammals. J. Dermatol. Sci. 49, 21–31. Nubel, U., Roumagnac, P., Feldkamp, M., Song, J.H., Ko, K.S., Huang, Y.C., Coombs, G., Ip, M., Westh, H., Skov, R., Struelens, M.J., Goering, R.V., Strommenger, B., Weller, A., Witte, W., Achtman, M., 2008. Frequent emergence and limited geographic dispersal of methicillin-resistant Staphylococcus aureus. Proc. Natl. Acad. Sci. USA 105, 14130–14135. Otto, M., 2010. Basis of virulence in community-associated methicillin-resistant Staphylococcus aureus. Annu. Rev. Microbiol. 64, 143–162. Patel, M., Hoesley, C.J., Moser, S.A., Stamm, A.M., Baddley, J.W., Waites, K.B., 2008. Dissemination of community-associated methicillin-resistant Staphylococcus aureus in a tertiary care hospital. South Med. J. 101, 40–45. Piechowicz, L., Garbacz, K., Wisniewska, K., Dabrowska-Szponar, M., 2011. Screening of Staphylococcus aureus nasal strains isolated from medical students for toxin genes. Folia Microbiol. (Praha) 56, 225–229. Proctor, R.A., 2012. Challenges for a universal Staphylococcus aureus vaccine. Clin. Infect. Dis. 54, 1179–1186. Rasigade, J.P., Laurent, F., Lina, G., Meugnier, H., Bes, M., Vandenesch, F., Etienne, J., Tristan, A., 2010. Global distribution and evolution of Panton-Valentine leukocidin-positive methicillin-susceptible Staphylococcus aureus, 1981– 2007. J. Infect. Dis. 201, 1589–1597. Rouzic, N., Janvier, F., Libert, N., Javouhey, E., Lina, G., Nizou, J.Y., Pasquier, P., Stamm, D., Brinquin, L., Pelletier, C., Vandenesch, F., Floret, D., Etienne, J., Gillet, Y., 2010. Prompt and successful toxin-targeting treatment of three patients with necrotizing pneumonia due to Staphylococcus aureus strains carrying the Panton-Valentine leukocidin genes. J. Clin. Microbiol. 48, 1952–1955. Schijffelen, M.J., Boel, C.H., Van Strijp, J.A., Fluit, A.C., 2010. Whole genome analysis of a livestock-associated methicillin-resistant Staphylococcus aureus ST398 isolate from a case of human endocarditis. BMC Genomics 11, 376. Schreiber, J.R., 2012. Role of Toll like receptors in the antibody response to encapsulated bacteria. Front Biosci. (Elite Ed) 4, 2638–2646. Seale, A., Finn, A., 2011. What is the best way to use conjugate vaccines? Curr. Opin. Infect. Dis. 24, 219–224. Sivaraman, K., Venkataraman, N., Cole, A.M., 2009. Staphylococcus aureus nasal carriage and its contributing factors. Future Microbiol. 4, 999–1008.
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Tacconelli, E., Carmeli, Y., Aizer, A., Ferreira, G., Foreman, M.G., D’agata, E.M., 2003. Mupirocin prophylaxis to prevent Staphylococcus aureus infection in patients undergoing dialysis: a meta-analysis. Clin. Infect. Dis. 37, 1629–1638. Tristan, A., Bes, M., Meugnier, H., Lina, G., Bozdogan, B., Courvalin, P., Reverdy, M.E., Enright, M.C., Vandenesch, F., Etienne, J., 2007. Global distribution of PantonValentine leukocidin–positive methicillin-resistant Staphylococcus aureus, 2006. Emerg. Infect. Dis. 13, 594–600. Vandenesch, F., Naimi, T., Enright, M.C., Lina, G., Nimmo, G.R., Heffernan, H., Liassine, N., Bes, M., Greenland, T., Reverdy, M.E., Etienne, J., 2003. Community-acquired methicillin-resistant Staphylococcus aureus carrying Panton-Valentine leukocidin genes: worldwide emergence. Emerg. Infect. Dis. 9, 978–984. Verkaik, N.J., Lebon, A., De Vogel, C.P., Hooijkaas, H., Verbrugh, H.A., Jaddoe, V.W., Hofman, A., Moll, H.A., Van Belkum, A., Van Wamel, W.J., 2010. Induction of antibodies by Staphylococcus aureus nasal colonization in young children. Clin. Microbiol. Infect. 16, 1312–1317. Von Eiff, C., Becker, K., Machka, K., Stammer, H., Peters, G., 2001. Nasal carriage as a source of Staphylococcus aureus bacteremia. Study group. N. Engl. J. Med. 344, 11–16. Wang, R., Braughton, K.R., Kretschmer, D., Bach, T.H., Queck, S.Y., Li, M., Kennedy, A.D., Dorward, D.W., Klebanoff, S.J., Peschel, A., Deleo, F.R., Otto, M., 2007. Identification of novel cytolytic peptides as key virulence determinants for community-associated MRSA. Nat. Med. 13, 1510–1514. Wertheim, H.F., Vos, M.C., Ott, A., Van Belkum, A., Voss, A., Kluytmans, J.A., Van Keulen, P.H., Vandenbroucke-Grauls, C.M., Meester, M.H., Verbrugh, H.A., 2004. Risk and outcome of nosocomial Staphylococcus aureus bacteraemia in nasal carriers versus non-carriers. Lancet 364, 703–705. Wiese, L., Mejer, N., Schonheyder, H.C., Westh, H., Jensen, A.G., Larsen, A.R., Skov, R., Benfield, T., 2013. A nationwide study of comorbidity and risk of reinfection after Staphylococcus aureus bacteraemia. J. Infect. 67, 199–205. Wyllie, D., Paul, J., Crook, D., 2011. Waves of trouble: MRSA strain dynamics and assessment of the impact of infection control. J. Antimicrob. Chemother. 66, 2685–2688. Yanagisawa, C., Hanaki, H., Natae, T., Sunakawa, K., 2007. Neutralization of staphylococcal exotoxins in vitro by human-origin intravenous immunoglobulin. J. Infect. Chemother. 13, 368–372. Yang, E.S., Tan, J., Eells, S., Rieg, G., Tagudar, G., Miller, L.G., 2010. Body site colonization in patients with community-associated methicillin-resistant Staphylococcus aureus and other types of S. aureus skin infections. Clin. Microbiol. Infect. 16, 425–431. Zielinska, A.K., Beenken, K.E., Joo, H.S., Mrak, L.N., Griffin, L.M., Luong, T.T., Lee, C.Y., Otto, M., Shaw, L.N., Smeltzer, M.S., 2011. Defining the strain-dependent impact of the Staphylococcal accessory regulator (sarA) on the alpha-toxin phenotype of Staphylococcus aureus. J. Bacteriol. 193, 2948–2958.
Please cite this article in press as: Rasigade, J.-P., Vandenesch, F. Staphylococcus aureus: A pathogen with still unresolved issues. Infect. Genet. Evol. (2013), http://dx.doi.org/10.1016/j.meegid.2013.08.018