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Literature highlights: Recent research in infectious disease drug resistance
Drug Resistance Updates 10 (2007) 68–75
Literature highlights
Recent research in infectious disease drug resistance
1. Bacterial infections Growth and antibiotic resistance of Mycobacterium avium in catheter biofilms Falkinham (Virginia Polytechnic Institute and State University, Blacksburg, USA) reports that cells of Mycobacterium avium adhered to plasticized polyvinyl chloride catheter tubing and grew at low nutrient concentration, consistent with reports of catheter-associated M. avium infection. Starting with initial cell densities of 1–2 × 106 cfu/ml, biofilms of approx. 350 cfu/cm formed within 24 h at room temperature. Growth rates were exponential for 1–2 week incubation (doubling time, 2.5 days) and by 4 weeks cell densities of 6.5 × 104 cfu/cm were reached. Cells grown in catheter biofilms were significantly more resistant to clarithromycin and rifamycin than cells grown in suspension. J. Med. Microbiol. 2007 February;56(Pt. 2):250–4. PMID: 17244808 [PubMed – in process]
(100%), 15 of 18 EMB-resistant isolates (83%), 18 of 30 STR-resistant isolates (60%), and 17 of 17 PZA-resistant isolates (100%) had mutations related to specific drug resistance. Eighteen of these mutations were novel and mapped as follows: one in rpoB, eight in katG, one in the mabA-inhA regulatory region, two in embB, five in pncA, and one in rrs. Escherichia coli isolates expressing individually five of the eight katG mutations showed loss of catalase and INH oxidation activities, and isolates carrying any of the five pncA mutations showed no pyrazinamidase activity, indicating that these mutations are associated with INH and PZA resistance, respectively. The sequencing-based method was also useful for testing sputa from tuberculosis patients and for screening of mutations in Mycobacterium bovis. J. Clin. Microbiol. 2007 January;45(1):179–92. [Epub 2006 November 15]. PMID: 17108078 [PubMed – indexed for MEDLINE] 1.1. Gram-positive bacteria
Detection of multidrug resistance in Mycobacterium tuberculosis
1.1.1. Staphylococcus aureus, Staphylococcus epidermidis Treatment options for invasive, multidrug-resistant Staphylococcus aureus infections
Sekiguchi and collagues (International Medical Center of Japan, Tokyo, Japan) describe a DNA sequencing-based method for the rapid detection of multiple mutations in the genome of drug-resistant Mycobacterium tuberculosis. They amplified simultaneously by PCR eight genome regions associated with drug resistance and determined the DNA sequences. The regions amplified were: rpoB for rifampin (RIF), katG and the mabA (fabG1)-inhA promoter for isoniazid (INH), embB for ethambutol (EMB), pncA for pyrazinamide (PZA), rpsL and rrs for streptomycin (STR), and gyrA for levofloxacin. Of the 138 clinical isolates tested, 55 were resistant to at least one drug. Thirty-four of 38 INH-resistant isolates (90%), 28 of 28 RIF-resistant isolates
Drew (Duke University School of Medicine, Durham, North Carolina) reviews treatment options for the treatment of serious, invasive infections caused by multidrug-resistant Staphylococcus aureus, particularly nosocomially acquired methicillin-resistant S. aureus (MRSA). Vancomycin is the gold standard for therapy of such infections, but reports of increasing in-vitro resistance to it underscore the need for alternative therapies. Older agents with favorable in vitro activity available in both oral and intravenous dose forms (trimethoprim-sulfamethoxazole and clindamycin) are being reexplored in the setting of community-acquired MRSA. Newer treatment options include linezolid, quinupristindalfopristin, daptomycin, and tigecycline. With the exception
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Literature highlights / Drug Resistance Updates 10 (2007) 68–75
of linezolid, these newer agents are intravenous. Combination therapy may be considered in invasive diseases refractory to standard monotherapies such as endocarditis, meningitis, and prosthetic device infections. Additional agents in late clinical development include the glycopeptides oritavancin, dalbavancin, telavancin and the cephalosporin ceftobiprole. Pharmacotherapy. 2007 February;27(2):227–49. PMID: 17253914 [PubMed – in process] Fitness cost of SCCmec in methicillin-resistant S. aureus by way of continuous culture Lee and colleagues (University of Otago, Dunedin, New Zealand and University of Zurich, Switzerland) describe the effect of introducing SCCmec elements type I or IV on the growth yield of S. aureus in glucose-limited continuous culture. Type I showed increased glucose consumption and ATP demand per gram of cells synthesized, and decreased cell yield compared to the parent strain. In contrast, type IV SCCmec had no adverse energetic effect explaining its broad spread. The findings offer an explanation for the prevalence of type IV SCCmec in the clinical setting. Antimicrob. Agents Chemother. 2007 February 5; [Epub ahead of print] PMID: 17283194 [PubMed – as supplied by publisher] Community-associated MRSA and healthcare risk factors A research team headed by Klevens, (Centers for Disease Control and Prevention, Atlanta, Georgia, USA) used surveillance data to determine frequency of MRSA infections caused by strains typically associated with community-acquired infections (USA300) among persons with healthcare-related risk factors (HRFs). Of patients with HRFs, 18%–28% had a “community-associated” strain, primarily USA300; of patients without HRFs, 26% had a “healthcare-associated” strain, typically USA100. Emerg. Infect. Dis. 2006 December;12(12):1991–3. PMID: 17326962 [PubMed – in process] The sesquiterpene farnesol inhibits recycling of the C55 lipid carrier of the murein monomer precursor leading to increased susceptibility of MRSA to -lactams The plant metabolite farnesol is known to potentiate antibacterial agents, though its mode of action is poorly understood. Kuroda and colleagues (University of Tsukuba, Ibaraki, Japan), investigated farnesol’s synergistic effects on commonly used antibacterials, -lactams in particular,
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and its potential inhibitory effects on cell wall synthesis. They examined farnesol’s effects on: (a) antimicrobial susceptibilities of methicillin-susceptible and -resistant S. aureus (MSSA and MRSA) to ampicillin, oxacillin, cefoxitin, bacitracin, teicoplanin, amikacin, ciprofloxacin and clarithromycin; (b) penicillin-binding protein PBP2 (2a) expression; (c) -lactamase secretion and activity; (d) staphyloxanthin production; and (e) cell wall synthesis. Farnesol increased susceptibility to all antibacterials tested except clarithromycin in both MSSA and MRSA. Although no apparent suppression of PBP2 expression was observed, -lactamase secretion and -lactamase activity were significantly reduced by farnesol. In addition, farnesol completely suppressed staphyloxanthin production. Finally, farnesol reduced the incorporation of N-acetylglucosamine (a cell wall precursor) and induced accumulation of C55 -PP, lipid I and lipid II. The authors conclude that farnesol increased -lactam susceptibility of MRSA by inhibiting cell wall biosynthesis through reduction of free C55 lipid carrier with subsequent retardation of murein monomer precursor transport across the cell membrane. J. Antimicrob. Chemother. 2007 January 22; [Epub ahead of print] PMID: 17242033 [PubMed – as supplied by publisher] Triclosan resistance in MRSA expressed as small colony variants: a novel mode of evasion of susceptibility to antiseptics Triclosan is widely used in domestic, commercial and healthcare settings, including reduction of MRSA load in carriers. Triclosan resistance is uncommon and is usually due to mutation in fabI or overexpression of efflux pumps. Bayston and colleagues (School of Medical and Surgical Sciences, University Hospitals Nottingham, UK) investigated the ability of triclosan-containing silicone elastomer to kill MRSA adherent to its surface. They exposed silicone discs containing triclosan to three strains of MRSA for 1 h, then were removed the discs at intervals, sonicated and analysed sonicates by chemiluminescence and viable counting. Viable counts fell until 51 h, when they began to increase, due to small colony variants (SCVs). They tested SCVs for known SCV characteristics and for susceptibility to triclosan. Of the three SCV strains, two showed impaired coagulase production and all showed reduced deoxyribonuclease production. None was auxotrophic. MICs of triclosan in the SCVs rose by between 8- and 67-fold. The authors conclude that prolonged exposure of MRSA to triclosan-impregnated silicone, as in ‘antimicrobial’ plastics or catheters, may result in the induction of SCVs and triclosan resistance. SCVs are pathogenic and persistent. New antibacterials with physiological targets similar to that of triclosan might also give rise to SCV resistance in clinical use.
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Literature highlights / Drug Resistance Updates 10 (2007) 68–75
J. Antimicrob. Chemother. 2007 March 2; [Epub ahead of print] PMID: 17337510 [PubMed – as supplied by publisher] Prevalence and clinical significance of S. aureus SCVs in cystic fibrosis lung disease S. aureus SCVs can be isolated from the chronically infected airways of patients suffering from cystic fibrosis (CF), though the clinical significance of this special phenotype in CF lung disease is unknown. Besier and colleagues (University Hospital of Frankfurt am Main, Germany) investigated the prevalence of S. aureus SCVs in CF lung disease in a 12-month prospective study and correlated the microbiological culture results with the patients’ clinical data. A total of 252 patients were screened for the presence of SCVs and a prevalence rate of 17% was found among S. aureus carriers. S. aureus isolates with the SCV phenotype showed significantly higher antibiotic resistance rates than those with the normal phenotype. Patients positive for SCVs were significantly older, more commonly co-colonized with Pseudomonas aeruginosa and showed signs of more advanced disease, such as lower forced expiratory volume in 1 s than patients harboring S. aureus with a normal phenotype. The logistic regression model determined lower weight, advanced age, and prior use of trimethoprim-sulfamethoxazole as independent risk factors for S. aureus SCV positivity. The prevalence of S. aureus SCVs in advanced CF lung disease points to a clinical significance on chronic and persistent infections associated with CF. J. Clin. Microbiol. 2007 January;45(1):168–72. [Epub 2006 Nov 15]. PMID: 17108072 [PubMed – indexed for MEDLINE]
1.1.2. Sreptococcus pneumoniae, Streptococcus mutans Geographically-based evaluation of multidrug resistance trends among Streptococcus pneumoniae in the USA: findings of the FAST surveillance initiative (2003–2004) Draghi DC, Jones ME, Sahm DF, Tillotson GS. Focus Bio-Inova Inc., 13665 Dulles Technology Drive, Suite 200, Herndon, VA 20171-4603, USA. Surveillance initiatives to track Streptococcus pneumoniae resistance trends allow updating the in vitro activity of available antimicrobial agents. The antimicrobial susceptibility profiles of S. pneumoniae (n = 1479 isolates) collected from 17 geographical areas across the USA (2003–2004) were analysed. Of the isolates, 37% were resistant to
one or more agents and 24% were resistant to two or more antibacterial classes (multidrug-resistant). Multidrug resistance involved resistance to -lactams, macrolides, tetracycline and trimethoprim/sulphamethoxazole, but rarely fluoroquinolones; over 96% of multidrug-resistant isolates were fluoroquinolone-susceptible. Multidrug resistance rates were prominent regardless of the geographical region surveyed. The authors warn that, as this trend continues, the empirical therapeutic options for S. pneumoniae infections will diminish and there will be a need to evaluate the effectiveness of newer, potent-but-flawed fluoroquinolones such as gemifloxacin. Int. J. Antimicrob. Agents. 2006 December;28(6):525–31. [Epub 2006 November 13]. PMID: 17101260 [PubMed – indexed for MEDLINE]
1.2. Gram-negative bacteria 1.2.1. Escherichia coli and other enterobacteria Functional analysis of replication and stability regions of broad-host-range conjugative plasmid CTX-M3 from the IncL/M incompatibility group Mierzejewska and colleagues (Institute of Biochemistry and Biophysics, PAS, Warsaw, Poland) report on a 89 kb conjugative plasmid, pCTX-M3, coding for a cefotaxime-hydrolysing -lactamase, and initially isolated from Citrobacter freundii in a Warsaw hospital. The plasmid conferred resistance to -lactams (ampicillin, piperacillin, ceftazidime, cefoxitin, cefotaxime) and to aminoglycosides (gentamicin and tobramycin). Similar conjugative plasmids were found in E. coli, Klebsiella pneumoniae, K. oxytoca, Serratia marcescens, Enterobacter cloacae and Morganella morgani in at least 15 hospitals in Poland suggesting quite a broad-host-range and a highly disseminative conjugation system. The authors found the plasmid to be a mosaic of cassettes originated from different replicons. Its replication system is almost 100% identical with that of pMU407.1 from the IncL/M incompatibility group isolated from K. pneumoniae, the conjugative operons share 50–60% homology mainly with pColIb-P9 (IncI) and include genes traL, M, N, identical to those found previously on the pACM1 plasmid from the IncM group. The stability systems are highly homologous to those from the NR1(R100) of IncFII group, pColIb-P9 or those from Yersinia plasmids. The research team established the broad-host-range for pCTX-M3 and defined its minireplicon in E. coli. They analyzed the role of stability cassettes and showed that the par operon consists of three orfs parA (stbA), parB (stbB) and nuc with a centromere-like region located upstream of the operon. Deletion of the par operon strongly destabilized pCTX-M3 despite the presence of the pemIK toxin–antidote system identical to that on
Literature highlights / Drug Resistance Updates 10 (2007) 68–75
NR1(R100) plasmids. Deletion of the pemIK operon had no effect on plasmid stability. Plasmid. 2007 March;57(2):95–107. [Epub 2006 November 7]. PMID: 17087993 [PubMed – in process] Wide geographic spread of diverse acquired AmpC -lactamases among E. coli and Klebsiella spp. in the UK and Ireland Woodford and colleagues (Health Protection Agency, London, UK) determined the distribution of acquired AmpC -lactamases in 173 isolates of E. coli and Klebsiella spp. submitted to the UK’s national reference laboratory for antibiotic resistance. Candidate isolates were those resistant to cefotaxime and/or ceftazidime, irrespective of addition of clavulanic acid. Genes encoding six phylogenetic groups of acquired AmpC enzymes were sought by PCR. Selected isolates were compared by pulsed-field gel electrophoresis (PFGE), and one bla(AmpC) amplicon was sequenced. Genes encoding acquired AmpC enzymes were detected in 67 (49%) candidate E. coli and 21 (55%) Klebsiella spp. Sixty isolates produced CIT-type enzymes, 14 had ACC types, 11 had FOX types and 3 had DHA enzymes. The low-level cephalosporin resistance of the remaining isolates (n = 85; 49%) was inferred to result from reduced permeability or, in E. coli, from overexpression of chromosomal ampC. Twenty-four E. coli isolates from one hospital produced a CIT-type enzyme, with 20 of these additionally producing a group 1 CTX-M extended-spectrum -lactamase (ESBL). PFGE indicated that these isolates belonged to UK epidemic strain A, which normally produces CTX-M-15, but no acquired AmpC. Sequencing a representative bla(AmpC) amplicon indicated that in one centre this strain had acquired a novel CMY-2 variant, designated CMY-23. The authors conclude that diverse acquired AmpC enzymes occur in E. coli and Klebsiella spp. isolates in the UK and Ireland, with CIT types the most common. Producers are geographically scattered, but with some local outbreaks. They further point out that acquisition of a CMY-2-like enzyme by E. coli epidemic strain A suggests that these enzymes may be poised to become an important public health issue. J. Antimicrob. Chemother. 2007 January;59(1):102–5. [Epub 2006 November 16]. PMID: 17110393 [PubMed – indexed for MEDLINE] Prevalence of plasmid-mediated quinolone resistance determinants, QnrA, QnrB, and QnrS, among clinical isolates of E. cloacae in a Taiwanese hospital Wu and colleagues (National Cheng Kung University and University Hospital, Tainan, Taiwan) determined by
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PCR and colony hybridization the prevalence of three plasmid-mediated quinolone resistance determinants, QnrA, QnrB, and QnrS, among 526 non-replicate clinical isolates of E. cloacae collected at a Taiwanese university hospital in 2004. The team also investigated the association of Qnr with the IMP-8 metallo--lactamase. Eighty-six (16%) of all isolates were qnr-positive, and the qnrA1-like, qnrB2-like, and qnrS1-like genes were detected alone or in combination in 3 (0.6%), 53 (10%), and 34 (6.5%) isolates, respectively. Among 149 putative ESBL-producing isolates, 59 (40%) isolates, all of which were SHV-12 producers, harbored qnrA (0.7%, 1 isolate), qnrB (29%, 43 isolates), or qnrS (12%, 18 isolates). Forty-four (78%) of 56 IMP-8 producers carried qnrB (59%, 33 isolates), qnrS (25%, 14 isolates), or both. PCR and sequence analysis revealed that qnrA1 was located in a complex sul1-type integron that contains dhr15, aadA2, qacE1, sul1, orf513, qnrA1, ampR, and qacE1. Conjugation experiments revealed coexistence of qnrB and blaIMP-8 on the transferred plasmids and the absence of -lactamase content on the transferred qnrS-positive plasmids. The transferred blaIMP-8 -positive plasmids with and without qnrB had very similar restriction patterns, suggesting horizontal mobility of qnrB. Pulsed-field gel electrophoresis showed 6 major patterns among the 44 qnr-positive IMP-8-producing isolates. The authors conclude that the extremely high prevalence of qnr among the metallo--lactamase-producing E. cloacae isolates in the hospital may be mainly due to intra-hospital spread of a few clones and dissemination of plasmids containing both qnrB and blaIMP-8 . Antimicrob. Agents. Chemother. 2007 January 22; [Epub ahead of print] PMID: 17242140 [PubMed – as supplied by publisher]
1.2.2. Other gram-negative bacteria Prevalence and characterization of class I integrons among Pseudomonas aeruginosa and Acinetobacter baumannii isolates from patients in Nanjing, China Gu B (First Affiliated Hospital of Nanjing Medical University, Nanjing, China) reported on the presence of Class I integrons in 40% (40/98) of Pseudomonas aeruginosa strains and 53% (56/106) of Acinetobacter baumannii strains in the Nanjing area of China, including several cassette arrays not previously reported. J. Clin. Microbiol. 2007 January;45(1):241–3. [Epub 2006 November 22]. PMID: 17122024 [PubMed – indexed for MEDLINE]
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Development and persistence of antimicrobial resistance in P. aeruginosa: a longitudinal observation in mechanically ventilated patients P. aeruginosa, a pathogen that readily acquires antimicrobial resistance against available antimicrobials, frequently colonizes intubated hospital patients and is subsequently responsible for ventilator-associated pneumonia (VAP). Reinhardt and colleagues (Centre Medical Universitaire de Geneve; and Hopital Universitaire de Geneve, Switzerland) investigated the rapidity of selection, as well as the persistence, of antimicrobial resistance and determined the underlying mechanisms. They selected 109 prospectively collected P. aeruginosa tracheal isolates from two patients based on their prolonged intubation and colonization period during which they had received carbapenem, fluoroquinolone (FQ) or combined -lactam-aminoglycoside therapies. They tested antimicrobial resistance phenotypes and used quantitative RealTime-PCR to measure expression of resistance determinants. Within 10 days after initiation of therapy, all treatment regimens selected resistant isolates. Resistance to -lactam and FQ was correlated with ampC and mexC gene expression levels, respectively, whereas imipenem resistance was attributable to decreased oprD expression. Combined -lactam-aminoglycoside resistance was associated with appearance of SCVs. Imipenem and FQ resistance persisted for prolonged time once the selecting antimicrobial treatment had been discontinued. In contrast, resistance to -lactams rapidly disappeared after removal of the selective pressure, to reappear promptly upon renewed exposure. The results suggest that resistant P. aeruginosa are selected in less than 10 days independently of the antimicrobial class. Different resistance mechanisms lead to loss or persistence of resistance after removal of the selecting agent. Even if resistant isolates are not evident on culture, these may persist in the lung and can rapidly be reselected. Antimicrob. Agents Chemother. 2007 January 29; [Epub ahead of print] PMID: 17261619 [PubMed – as supplied by publisher]
also address possible mechanisms of resistance and pharmacokinetic limitations of the drug. The isolates underwent PFGE and exposure to phenyl-arginine--naphthylamide (PAbetaN), an efflux pump inhibitor. Patient 1 died of overwhelming A. baumannii infection and Patient 2 recovered after a change in antibiotic therapy. The MICs of tigecycline were 4 and 16 g/ml, respectively. Both isolates had a multidrug-resistant (MDR) phenotype and were genotypically unrelated. After exposure to PAbetaN, the MICs reduced to 1 and 4 g/ml, respectively. This is the first clinical description of bloodstream infection caused by tigecycline-non-susceptible A. baumannii. Such resistance appears to be at least partly attributable to an efflux pump mechanism. The authors urge caution when using tigecycline for treatment of A. baumannii bloodstream infection, given the reported low serum levels of this drug. J. Antimicrob. Chemother. 2007 January;59(1):128–31. [Epub 2006 November 1]. PMID: 17082201 [PubMed – indexed for MEDLINE] High tigecycline resistance in multidrug-resistant A. baumannii Carmeli and colleagues (Sackler Faculty of Medicine, Tel Aviv University, Israel) evaluated the in vitro activity of tigecycline against 82 non-replicate clinical isolates of MDR A. baumannii collected before the introduction of tigecycline. The majority (66%) of the isolates were resistant to tigecycline, 12% were intermediate and 22% were susceptible. The MIC50 and MIC90 of tigecycline were 16 and 32 g/ml, respectively, with a wide MIC range of 1–128 g/ml. The authors report high resistance rates to tigecycline, and higher than previously described MICs, in multiple clones of MDR A. baumannii. Their findings have disturbing clinical implications, as tigecycline represents a new treatment choice for infections caused by A. baumannii. J. Antimicrob. Chemother. 2007 March 12; [Epub ahead of print] PMID: 17353223 [PubMed – as supplied by publisher]
A. baumannii bloodstream infection while receiving tigecycline: a cautionary report Tigecycline has shown in vitro activity against A. baumannii, though published clinical experience with tigecycline outside clinical trials is lacking. A. baumannii is an opportunistic, often multiresistant gram-negative bacillus of increasing medical importance that can cause healthcareassociated infections and can survive for prolonged periods in the environment and on the hands of healthcare workers. Paterson and collegues (University of Pittsburgh Medical Center, Pittsburgh, PA, USA) describe bloodstream infections caused by tigecycline-non-susceptible A. baumannii in two patients receiving tigecycline for other indications. They
Multidrug-resistant Acinetobacter infection mortality rate and length of hospitalization The proportion of healthcare-associated infections caused by Acinetobacter spp. has increased over the past decade in the United States and worldwide. Sunenshine and colleagues (Centers for Disease Control and Prevention, Atlanta, Georgia, USA) performed a retrospective, matched cohort investigation at two Baltimore hospitals to examine outcomes of patients with MDR Acinetobacter infection compared with patients with susceptible Acinetobacter infections and patients without Acinetobacter infections. Multivariable analysis controlling for severity of illness and
Literature highlights / Drug Resistance Updates 10 (2007) 68–75
underlying disease identified an independent association between patients with MDR Acinetobacter infection (n = 96) and increased hospital and intensive care unit length of stay compared with 91 patients with susceptible Acinetobacter infection and 89 uninfected patients. Increased hospitalization associated with MDR Acinetobacter infection emphasizes the need for infection control strategies to prevent cross-transmission in healthcare settings. Emerg. Infect. Dis. 2007;13(1)2007, Posted 02/16/2007 Differentiation and distribution of colistin- and sodium dodecyl sulfate-tolerant cells in P. aeruginosa biofilms During P. aeruginosa flow cell biofilm development, the cell population differentiates into a nonmotile subpopulation which forms microcolonies and a migrating subpopulation which eventually colonizes the top of the microcolonies, resulting in the development of mushroom-shaped multicellular structures. Haagensen and colleagues (The Technical University of Denmark, Lyngby, Denmark) found that the cap-forming subpopulation developed tolerance to membrane-targeting antimicrobial agents, such as the cyclic cationic peptide colistin and the detergent sodium dodecyl sulfate. The stalk-forming subpopulation, on the other hand, was sensitive to the membrane-targeting antibacterial agents. All biofilm-associated cells were sensitive to the antibacterial agents when tested in standard plate assays. A mutation eliminating the production of type IV pili, and hence surface-associated motility, prevented the formation of regular mushroom-shaped structures in the flow cell biofilms; the development of tolerance to the antimicrobial agents was found to be affected as well. Mutations in genes interfering with lipopolysaccharide modification (pmr) eliminated the biofilm-associated colistin tolerance phenotype. Experiments with a PAO1 strain harboring a pmr–gfp fusion showed that only the cap-forming subpopulation in biofilms treated with colistin expresses the pmr operon. The authors suggest that increased antibiotic tolerance in biofilms is a consequence of differentiation into distinct subpopulations with different phenotypic properties. J. Bacteriol. 2007 January; 189(1):28–37. [Epub 2006 October 13] PMID: 17041046 [PubMed – indexed for MEDLINE]
2. Fungal infections Evaluating prophylaxis of invasive fungal infections in patients with haematologic malignancies Patients with hematologic malignancies are at substantial risk of developing invasive fungal infections (IFI) that
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are associated with substantial morbidity and mortality. Maertens (University Hospital Gasthuisberg, Leuven, Belgium) reviews the epidemiology, risk factors and efficacy of antifungal prophylaxis in patients with hematologic malignancies. The epidemiology of IFI has changed substantially in recent years with Candida albicans becoming less common owing to the widespread prophylactic use of azole antifungals. Invasive aspergillosis, fusariosis, and zygomycosis have increased in frequency. This change is at least partly related to the use of broad-spectrum antifungal agents, either as prophylaxis or as empirical treatment. Other risk factors for IFI include prior fungal exposure, immunosuppression, underlying disease, graft-versus-host disease, and organ dysfunction. Inconsistent results have been reported in studies evaluating the efficacy of antifungal prophylaxis in patients at risk of IFI. Meta-analyses found that antifungals, such as fluconazole and itraconazole, are effective in decreasing IFI and IFI-related mortality, primarily owing to yeast infections in patients with more severe immunosuppression (i.e. patients undergoing bone marrow transplantation), but do not decrease the overall mortality. The European Conference on Infections in Leukemia (ECIL) guidelines currently recommend fluconazole (AI, ie. strongly recommended, based on at least 1 well-executed, randomized trial) and itraconazole (BI, ie. generally recommended, based on at least one well-executed, randomized trial) in allogeneic transplant recipients. Posaconazole, a triazole antifungal, has been recently shown to decrease IFI incidence and overall mortality in some high-risk patients compared with standard azoles. Based on preliminary data, a provisional AI ECIL recommendation has been given. Eur. J. Haematol. 2007 January 23; [Epub ahead of print] PMID: 17241370 [PubMed – as supplied by publisher] The concentration-dependent nature of in vitro amphotericin B-itraconazole interaction against Aspergillus fumigatus: isobolographic and response surface analysis of complex pharmacodynamic interactions Meletiadis and colleagues (National Cancer Institute, Pediatric Oncology Branch, Bethesda, MD, USA) explored the in vitro combination of amphotericin B with itraconazole against 14 clinical isolates of Aspergillus fumigatus (9 itraconazole susceptible and 5 itraconazole resistant) with a colorimetric broth microdilution checkerboard technique using two drug interaction models able to explore complicated patterns of interactions: the response surface analysis of Bliss independence and the isobolographic analysis of Loewe additivity zero interaction theories. Synergy was found at combinations with low concentrations of amphotericin B (<0.125 g/ml), whereas antagonism was found at combinations with higher concentrations of amphotericin B. For itraconazole-resistant isolates, synergistic interactions were observed at high concentrations of
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Literature highlights / Drug Resistance Updates 10 (2007) 68–75
itraconazole (>0.5 g/ml). Synergy was more frequently observed for the itraconazole-resistant isolates than for the itraconazole-susceptible isolates. Int. J. Antimicrob. Agents. 2006 November;28(5):439–49. [Epub 2006 October 19]. PMID: 17055706 [PubMed – indexed for MEDLINE] Multi-azole resistance in Aspergillus fumigatus Howard and colleagues (The University of Manchester, Manchester, UK) report multi-azole resistance in Aspergillus fumigatus from a patient who received long-term treatment with itraconazole and voriconazole for bilateral chronic cavitary aspergillosis with aspergillomas and whose isolates of A. fumigatus developed simultaneous resistance to itraconazole and voriconazole. A novel mutation (G138C) in the target gene (cyp51A) encoding 14␣-demethylase was detected. The patient had some response to intravenous caspofungin, which he received six times weekly, without the development of resistance over 9 months. Int. J. Antimicrob. Agents. 2006 November;28(5):450–3. [Epub 2006 October 10]. PMID: 17034993 [PubMed – indexed for MEDLINE] Use of fluconazole as a surrogate marker to predict susceptibility and resistance to voriconazole among 13,338 clinical isolates of Candida spp Voriconazole is a new triazole antifungal with an expanded spectrum and potency against Candida spp., Aspergillus spp., and other opportunistic fungal pathogens. Pfaller and colleagues (University of Iowa College of Medicine, Iowa City, IA, USA) carried out a study to addresses the use of fluconazole as a surrogate marker to predict the susceptibility of Candida spp. to voriconazole. Reference broth microdilution MIC results for 13,338 strains of Candida spp. isolated from more than 200 medical centers worldwide were used. Voriconazole MICs and interpretive categories (susceptible, ≤1 g/ml; susceptible dose dependent, 2 g/ml; resistant, ≥4 g/ml) were compared with those of fluconazole by regression statistics and error rate bounding analyses. For all 13,338 isolates, the absolute categorical agreement was 92% (false susceptible or very major error [VME], 0.0%). Since voriconazole is 16- to 32-fold more potent than fluconazole, the performance of fluconazole as a surrogate marker for voriconazole susceptibility was improved by designating those isolates with fluconazole MICs of ≤32 g/ml as being susceptible to voriconazole, resulting in a categorical agreement of 97% with 0.1% VME. The authors recommend that clinical laboratories performing antifungal susceptibility testing of fluconazole against Candida spp. use these results as surrogate markers until commercial FDA-approved voriconazole susceptibility tests become available.
J. Clin. Microbiol. 2007 January;45(1):70–5. [Epub 2006 November 1]. PMID: 17079501 [PubMed – indexed for MEDLINE] Posaconazole for the treatment of azole-refractory oropharyngeal and esophageal candidiasis in subjects with HIV infection Skiest and colleagues (University of Texas Southwestern Medical Center, Dallas, Texas, USA) evaluated the efficacy and safety of oral posaconazole for human immunodeficiency virus (HIV)-infected patients with oropharyngeal candidiasis (OPC) and/or esophageal candidiasis (EC) who were clinically refractory to treatment with oral fluconazole or itraconazole. Subjects received either oral posaconazole (400 mg twice daily) for 3 days followed by oral posaconazole (400 mg once daily) for 25 days (regimen A; 103 patients) or oral posaconazole (400 mg twice daily) for 28 days (regimen B; 96 patients). The primary end point was cure or improvement after 28 days. Primary efficacy analyses were performed on the subset of treated subjects with refractory disease (e.g., baseline culture positive for fluconazole- or itraconazole-resistant Candida species or persistent or progressive clinical signs or symptoms consistent with treatment failure). Of the modified intent-to-treat population, 132 (75%) of 176 subjects achieved a clinical response to posaconazole treatment. Clinical response rates were similar between regimen A recipients (75%) and regimen B recipients (75%). Clinical responses occurred in 67 (73%) of 92 subjects with baseline isolates resistant to fluconazole, 49 (74%) of 66 subjects with baseline isolates resistant to itraconazole, and 42 (74%) of 57 subjects with isolates resistant to both. Clinical response was achieved in 32 (74%) of 43 subjects with endoscopically documented EC. The most common treatment-related adverse events were diarrhea (11%), neutropenia (7%), flatulence (6%), and nausea (6%). Eight subjects (4%) discontinued therapy as a result of a treatmentrelated adverse event. Clin. Infect. Dis. 2007 February 15;44(4):607–14. [Epub 2007 January 17] PMID: 17243069 [PubMed – indexed for MEDLINE]
3. Viral infections 3.1. Influenza virus Antiviral resistance and the control of pandemic influenza The response to the next influenza pandemic will likely include extensive use of antiviral drugs (mainly oseltamivir), combined with other transmission-reducing measures. Ani-
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mal and in vitro studies suggest that some strains of influenza may become resistant to oseltamivir while maintaining infectiousness (fitness). Use of antiviral agents on the scale anticipated for the control of pandemic influenza will create an unprecedented selective pressure for the emergence and spread of these strains. Nonetheless, antiviral resistance has received little attention when evaluating these plans. Lipsitch and colleagues (Harvard School of Public Health, Boston, Massachusetts; Massachusetts General Hospital, Boston, Massachusetts; and Emory University, Atlanta, Georgia, USA) designed and analyzed a deterministic compartmental model of the transmission of oseltamivir-sensitive and -resistant influenza infections during a pandemic. The model predicts that even if antiviral treatment or prophylaxis leads to the emergence of a transmissible resistant strain in as few as 1 in 50,000 treated persons and 1 in 500,000 prophylaxed persons, widespread use of antivirals may strongly promote the spread of resistant strains at the population level, leading to a prevalence of tens of percent by the end of a pandemic. On the other hand, even in cir-
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cumstances in which a resistant strain spreads widely, the use of antivirals may significantly delay and/or reduce the total size of the pandemic. If resistant strains carry some fitness cost, then, despite widespread emergence of resistance, antivirals could slow pandemic spread by months or more and buy time for vaccine development; this delay would be prolonged by nondrug control measures (such as social distancing) that reduce transmission, or use of a stockpiled suboptimal vaccine. Surprisingly, the model suggests that such nondrug control measures would increase the proportion of the epidemic caused by resistant strains. The authors conclude that the benefits of antiviral drug use to control an influenza pandemic may be reduced, although not completely offset, by drug resistance in the virus. They recommend that the risk of resistance should be considered in pandemic planning and monitored closely during a pandemic. PLoS Med. 2007 January 23;4(1):e15 [Epub ahead of print] PMID: 17253900 [PubMed – as supplied by publisher]
Literature highlights
Recent research in infectious disease drug resistance
1. Bacterial infections Growth and antibiotic resistance of Mycobacterium avium in catheter biofilms Falkinham (Virginia Polytechnic Institute and State University, Blacksburg, USA) reports that cells of Mycobacterium avium adhered to plasticized polyvinyl chloride catheter tubing and grew at low nutrient concentration, consistent with reports of catheter-associated M. avium infection. Starting with initial cell densities of 1–2 × 106 cfu/ml, biofilms of approx. 350 cfu/cm formed within 24 h at room temperature. Growth rates were exponential for 1–2 week incubation (doubling time, 2.5 days) and by 4 weeks cell densities of 6.5 × 104 cfu/cm were reached. Cells grown in catheter biofilms were significantly more resistant to clarithromycin and rifamycin than cells grown in suspension. J. Med. Microbiol. 2007 February;56(Pt. 2):250–4. PMID: 17244808 [PubMed – in process]
(100%), 15 of 18 EMB-resistant isolates (83%), 18 of 30 STR-resistant isolates (60%), and 17 of 17 PZA-resistant isolates (100%) had mutations related to specific drug resistance. Eighteen of these mutations were novel and mapped as follows: one in rpoB, eight in katG, one in the mabA-inhA regulatory region, two in embB, five in pncA, and one in rrs. Escherichia coli isolates expressing individually five of the eight katG mutations showed loss of catalase and INH oxidation activities, and isolates carrying any of the five pncA mutations showed no pyrazinamidase activity, indicating that these mutations are associated with INH and PZA resistance, respectively. The sequencing-based method was also useful for testing sputa from tuberculosis patients and for screening of mutations in Mycobacterium bovis. J. Clin. Microbiol. 2007 January;45(1):179–92. [Epub 2006 November 15]. PMID: 17108078 [PubMed – indexed for MEDLINE] 1.1. Gram-positive bacteria
Detection of multidrug resistance in Mycobacterium tuberculosis
1.1.1. Staphylococcus aureus, Staphylococcus epidermidis Treatment options for invasive, multidrug-resistant Staphylococcus aureus infections
Sekiguchi and collagues (International Medical Center of Japan, Tokyo, Japan) describe a DNA sequencing-based method for the rapid detection of multiple mutations in the genome of drug-resistant Mycobacterium tuberculosis. They amplified simultaneously by PCR eight genome regions associated with drug resistance and determined the DNA sequences. The regions amplified were: rpoB for rifampin (RIF), katG and the mabA (fabG1)-inhA promoter for isoniazid (INH), embB for ethambutol (EMB), pncA for pyrazinamide (PZA), rpsL and rrs for streptomycin (STR), and gyrA for levofloxacin. Of the 138 clinical isolates tested, 55 were resistant to at least one drug. Thirty-four of 38 INH-resistant isolates (90%), 28 of 28 RIF-resistant isolates
Drew (Duke University School of Medicine, Durham, North Carolina) reviews treatment options for the treatment of serious, invasive infections caused by multidrug-resistant Staphylococcus aureus, particularly nosocomially acquired methicillin-resistant S. aureus (MRSA). Vancomycin is the gold standard for therapy of such infections, but reports of increasing in-vitro resistance to it underscore the need for alternative therapies. Older agents with favorable in vitro activity available in both oral and intravenous dose forms (trimethoprim-sulfamethoxazole and clindamycin) are being reexplored in the setting of community-acquired MRSA. Newer treatment options include linezolid, quinupristindalfopristin, daptomycin, and tigecycline. With the exception
1368-7646/$ – see front matter doi:10.1016/j.drup.2007.03.002
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of linezolid, these newer agents are intravenous. Combination therapy may be considered in invasive diseases refractory to standard monotherapies such as endocarditis, meningitis, and prosthetic device infections. Additional agents in late clinical development include the glycopeptides oritavancin, dalbavancin, telavancin and the cephalosporin ceftobiprole. Pharmacotherapy. 2007 February;27(2):227–49. PMID: 17253914 [PubMed – in process] Fitness cost of SCCmec in methicillin-resistant S. aureus by way of continuous culture Lee and colleagues (University of Otago, Dunedin, New Zealand and University of Zurich, Switzerland) describe the effect of introducing SCCmec elements type I or IV on the growth yield of S. aureus in glucose-limited continuous culture. Type I showed increased glucose consumption and ATP demand per gram of cells synthesized, and decreased cell yield compared to the parent strain. In contrast, type IV SCCmec had no adverse energetic effect explaining its broad spread. The findings offer an explanation for the prevalence of type IV SCCmec in the clinical setting. Antimicrob. Agents Chemother. 2007 February 5; [Epub ahead of print] PMID: 17283194 [PubMed – as supplied by publisher] Community-associated MRSA and healthcare risk factors A research team headed by Klevens, (Centers for Disease Control and Prevention, Atlanta, Georgia, USA) used surveillance data to determine frequency of MRSA infections caused by strains typically associated with community-acquired infections (USA300) among persons with healthcare-related risk factors (HRFs). Of patients with HRFs, 18%–28% had a “community-associated” strain, primarily USA300; of patients without HRFs, 26% had a “healthcare-associated” strain, typically USA100. Emerg. Infect. Dis. 2006 December;12(12):1991–3. PMID: 17326962 [PubMed – in process] The sesquiterpene farnesol inhibits recycling of the C55 lipid carrier of the murein monomer precursor leading to increased susceptibility of MRSA to -lactams The plant metabolite farnesol is known to potentiate antibacterial agents, though its mode of action is poorly understood. Kuroda and colleagues (University of Tsukuba, Ibaraki, Japan), investigated farnesol’s synergistic effects on commonly used antibacterials, -lactams in particular,
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and its potential inhibitory effects on cell wall synthesis. They examined farnesol’s effects on: (a) antimicrobial susceptibilities of methicillin-susceptible and -resistant S. aureus (MSSA and MRSA) to ampicillin, oxacillin, cefoxitin, bacitracin, teicoplanin, amikacin, ciprofloxacin and clarithromycin; (b) penicillin-binding protein PBP2 (2a) expression; (c) -lactamase secretion and activity; (d) staphyloxanthin production; and (e) cell wall synthesis. Farnesol increased susceptibility to all antibacterials tested except clarithromycin in both MSSA and MRSA. Although no apparent suppression of PBP2 expression was observed, -lactamase secretion and -lactamase activity were significantly reduced by farnesol. In addition, farnesol completely suppressed staphyloxanthin production. Finally, farnesol reduced the incorporation of N-acetylglucosamine (a cell wall precursor) and induced accumulation of C55 -PP, lipid I and lipid II. The authors conclude that farnesol increased -lactam susceptibility of MRSA by inhibiting cell wall biosynthesis through reduction of free C55 lipid carrier with subsequent retardation of murein monomer precursor transport across the cell membrane. J. Antimicrob. Chemother. 2007 January 22; [Epub ahead of print] PMID: 17242033 [PubMed – as supplied by publisher] Triclosan resistance in MRSA expressed as small colony variants: a novel mode of evasion of susceptibility to antiseptics Triclosan is widely used in domestic, commercial and healthcare settings, including reduction of MRSA load in carriers. Triclosan resistance is uncommon and is usually due to mutation in fabI or overexpression of efflux pumps. Bayston and colleagues (School of Medical and Surgical Sciences, University Hospitals Nottingham, UK) investigated the ability of triclosan-containing silicone elastomer to kill MRSA adherent to its surface. They exposed silicone discs containing triclosan to three strains of MRSA for 1 h, then were removed the discs at intervals, sonicated and analysed sonicates by chemiluminescence and viable counting. Viable counts fell until 51 h, when they began to increase, due to small colony variants (SCVs). They tested SCVs for known SCV characteristics and for susceptibility to triclosan. Of the three SCV strains, two showed impaired coagulase production and all showed reduced deoxyribonuclease production. None was auxotrophic. MICs of triclosan in the SCVs rose by between 8- and 67-fold. The authors conclude that prolonged exposure of MRSA to triclosan-impregnated silicone, as in ‘antimicrobial’ plastics or catheters, may result in the induction of SCVs and triclosan resistance. SCVs are pathogenic and persistent. New antibacterials with physiological targets similar to that of triclosan might also give rise to SCV resistance in clinical use.
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J. Antimicrob. Chemother. 2007 March 2; [Epub ahead of print] PMID: 17337510 [PubMed – as supplied by publisher] Prevalence and clinical significance of S. aureus SCVs in cystic fibrosis lung disease S. aureus SCVs can be isolated from the chronically infected airways of patients suffering from cystic fibrosis (CF), though the clinical significance of this special phenotype in CF lung disease is unknown. Besier and colleagues (University Hospital of Frankfurt am Main, Germany) investigated the prevalence of S. aureus SCVs in CF lung disease in a 12-month prospective study and correlated the microbiological culture results with the patients’ clinical data. A total of 252 patients were screened for the presence of SCVs and a prevalence rate of 17% was found among S. aureus carriers. S. aureus isolates with the SCV phenotype showed significantly higher antibiotic resistance rates than those with the normal phenotype. Patients positive for SCVs were significantly older, more commonly co-colonized with Pseudomonas aeruginosa and showed signs of more advanced disease, such as lower forced expiratory volume in 1 s than patients harboring S. aureus with a normal phenotype. The logistic regression model determined lower weight, advanced age, and prior use of trimethoprim-sulfamethoxazole as independent risk factors for S. aureus SCV positivity. The prevalence of S. aureus SCVs in advanced CF lung disease points to a clinical significance on chronic and persistent infections associated with CF. J. Clin. Microbiol. 2007 January;45(1):168–72. [Epub 2006 Nov 15]. PMID: 17108072 [PubMed – indexed for MEDLINE]
1.1.2. Sreptococcus pneumoniae, Streptococcus mutans Geographically-based evaluation of multidrug resistance trends among Streptococcus pneumoniae in the USA: findings of the FAST surveillance initiative (2003–2004) Draghi DC, Jones ME, Sahm DF, Tillotson GS. Focus Bio-Inova Inc., 13665 Dulles Technology Drive, Suite 200, Herndon, VA 20171-4603, USA. Surveillance initiatives to track Streptococcus pneumoniae resistance trends allow updating the in vitro activity of available antimicrobial agents. The antimicrobial susceptibility profiles of S. pneumoniae (n = 1479 isolates) collected from 17 geographical areas across the USA (2003–2004) were analysed. Of the isolates, 37% were resistant to
one or more agents and 24% were resistant to two or more antibacterial classes (multidrug-resistant). Multidrug resistance involved resistance to -lactams, macrolides, tetracycline and trimethoprim/sulphamethoxazole, but rarely fluoroquinolones; over 96% of multidrug-resistant isolates were fluoroquinolone-susceptible. Multidrug resistance rates were prominent regardless of the geographical region surveyed. The authors warn that, as this trend continues, the empirical therapeutic options for S. pneumoniae infections will diminish and there will be a need to evaluate the effectiveness of newer, potent-but-flawed fluoroquinolones such as gemifloxacin. Int. J. Antimicrob. Agents. 2006 December;28(6):525–31. [Epub 2006 November 13]. PMID: 17101260 [PubMed – indexed for MEDLINE]
1.2. Gram-negative bacteria 1.2.1. Escherichia coli and other enterobacteria Functional analysis of replication and stability regions of broad-host-range conjugative plasmid CTX-M3 from the IncL/M incompatibility group Mierzejewska and colleagues (Institute of Biochemistry and Biophysics, PAS, Warsaw, Poland) report on a 89 kb conjugative plasmid, pCTX-M3, coding for a cefotaxime-hydrolysing -lactamase, and initially isolated from Citrobacter freundii in a Warsaw hospital. The plasmid conferred resistance to -lactams (ampicillin, piperacillin, ceftazidime, cefoxitin, cefotaxime) and to aminoglycosides (gentamicin and tobramycin). Similar conjugative plasmids were found in E. coli, Klebsiella pneumoniae, K. oxytoca, Serratia marcescens, Enterobacter cloacae and Morganella morgani in at least 15 hospitals in Poland suggesting quite a broad-host-range and a highly disseminative conjugation system. The authors found the plasmid to be a mosaic of cassettes originated from different replicons. Its replication system is almost 100% identical with that of pMU407.1 from the IncL/M incompatibility group isolated from K. pneumoniae, the conjugative operons share 50–60% homology mainly with pColIb-P9 (IncI) and include genes traL, M, N, identical to those found previously on the pACM1 plasmid from the IncM group. The stability systems are highly homologous to those from the NR1(R100) of IncFII group, pColIb-P9 or those from Yersinia plasmids. The research team established the broad-host-range for pCTX-M3 and defined its minireplicon in E. coli. They analyzed the role of stability cassettes and showed that the par operon consists of three orfs parA (stbA), parB (stbB) and nuc with a centromere-like region located upstream of the operon. Deletion of the par operon strongly destabilized pCTX-M3 despite the presence of the pemIK toxin–antidote system identical to that on
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NR1(R100) plasmids. Deletion of the pemIK operon had no effect on plasmid stability. Plasmid. 2007 March;57(2):95–107. [Epub 2006 November 7]. PMID: 17087993 [PubMed – in process] Wide geographic spread of diverse acquired AmpC -lactamases among E. coli and Klebsiella spp. in the UK and Ireland Woodford and colleagues (Health Protection Agency, London, UK) determined the distribution of acquired AmpC -lactamases in 173 isolates of E. coli and Klebsiella spp. submitted to the UK’s national reference laboratory for antibiotic resistance. Candidate isolates were those resistant to cefotaxime and/or ceftazidime, irrespective of addition of clavulanic acid. Genes encoding six phylogenetic groups of acquired AmpC enzymes were sought by PCR. Selected isolates were compared by pulsed-field gel electrophoresis (PFGE), and one bla(AmpC) amplicon was sequenced. Genes encoding acquired AmpC enzymes were detected in 67 (49%) candidate E. coli and 21 (55%) Klebsiella spp. Sixty isolates produced CIT-type enzymes, 14 had ACC types, 11 had FOX types and 3 had DHA enzymes. The low-level cephalosporin resistance of the remaining isolates (n = 85; 49%) was inferred to result from reduced permeability or, in E. coli, from overexpression of chromosomal ampC. Twenty-four E. coli isolates from one hospital produced a CIT-type enzyme, with 20 of these additionally producing a group 1 CTX-M extended-spectrum -lactamase (ESBL). PFGE indicated that these isolates belonged to UK epidemic strain A, which normally produces CTX-M-15, but no acquired AmpC. Sequencing a representative bla(AmpC) amplicon indicated that in one centre this strain had acquired a novel CMY-2 variant, designated CMY-23. The authors conclude that diverse acquired AmpC enzymes occur in E. coli and Klebsiella spp. isolates in the UK and Ireland, with CIT types the most common. Producers are geographically scattered, but with some local outbreaks. They further point out that acquisition of a CMY-2-like enzyme by E. coli epidemic strain A suggests that these enzymes may be poised to become an important public health issue. J. Antimicrob. Chemother. 2007 January;59(1):102–5. [Epub 2006 November 16]. PMID: 17110393 [PubMed – indexed for MEDLINE] Prevalence of plasmid-mediated quinolone resistance determinants, QnrA, QnrB, and QnrS, among clinical isolates of E. cloacae in a Taiwanese hospital Wu and colleagues (National Cheng Kung University and University Hospital, Tainan, Taiwan) determined by
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PCR and colony hybridization the prevalence of three plasmid-mediated quinolone resistance determinants, QnrA, QnrB, and QnrS, among 526 non-replicate clinical isolates of E. cloacae collected at a Taiwanese university hospital in 2004. The team also investigated the association of Qnr with the IMP-8 metallo--lactamase. Eighty-six (16%) of all isolates were qnr-positive, and the qnrA1-like, qnrB2-like, and qnrS1-like genes were detected alone or in combination in 3 (0.6%), 53 (10%), and 34 (6.5%) isolates, respectively. Among 149 putative ESBL-producing isolates, 59 (40%) isolates, all of which were SHV-12 producers, harbored qnrA (0.7%, 1 isolate), qnrB (29%, 43 isolates), or qnrS (12%, 18 isolates). Forty-four (78%) of 56 IMP-8 producers carried qnrB (59%, 33 isolates), qnrS (25%, 14 isolates), or both. PCR and sequence analysis revealed that qnrA1 was located in a complex sul1-type integron that contains dhr15, aadA2, qacE1, sul1, orf513, qnrA1, ampR, and qacE1. Conjugation experiments revealed coexistence of qnrB and blaIMP-8 on the transferred plasmids and the absence of -lactamase content on the transferred qnrS-positive plasmids. The transferred blaIMP-8 -positive plasmids with and without qnrB had very similar restriction patterns, suggesting horizontal mobility of qnrB. Pulsed-field gel electrophoresis showed 6 major patterns among the 44 qnr-positive IMP-8-producing isolates. The authors conclude that the extremely high prevalence of qnr among the metallo--lactamase-producing E. cloacae isolates in the hospital may be mainly due to intra-hospital spread of a few clones and dissemination of plasmids containing both qnrB and blaIMP-8 . Antimicrob. Agents. Chemother. 2007 January 22; [Epub ahead of print] PMID: 17242140 [PubMed – as supplied by publisher]
1.2.2. Other gram-negative bacteria Prevalence and characterization of class I integrons among Pseudomonas aeruginosa and Acinetobacter baumannii isolates from patients in Nanjing, China Gu B (First Affiliated Hospital of Nanjing Medical University, Nanjing, China) reported on the presence of Class I integrons in 40% (40/98) of Pseudomonas aeruginosa strains and 53% (56/106) of Acinetobacter baumannii strains in the Nanjing area of China, including several cassette arrays not previously reported. J. Clin. Microbiol. 2007 January;45(1):241–3. [Epub 2006 November 22]. PMID: 17122024 [PubMed – indexed for MEDLINE]
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Development and persistence of antimicrobial resistance in P. aeruginosa: a longitudinal observation in mechanically ventilated patients P. aeruginosa, a pathogen that readily acquires antimicrobial resistance against available antimicrobials, frequently colonizes intubated hospital patients and is subsequently responsible for ventilator-associated pneumonia (VAP). Reinhardt and colleagues (Centre Medical Universitaire de Geneve; and Hopital Universitaire de Geneve, Switzerland) investigated the rapidity of selection, as well as the persistence, of antimicrobial resistance and determined the underlying mechanisms. They selected 109 prospectively collected P. aeruginosa tracheal isolates from two patients based on their prolonged intubation and colonization period during which they had received carbapenem, fluoroquinolone (FQ) or combined -lactam-aminoglycoside therapies. They tested antimicrobial resistance phenotypes and used quantitative RealTime-PCR to measure expression of resistance determinants. Within 10 days after initiation of therapy, all treatment regimens selected resistant isolates. Resistance to -lactam and FQ was correlated with ampC and mexC gene expression levels, respectively, whereas imipenem resistance was attributable to decreased oprD expression. Combined -lactam-aminoglycoside resistance was associated with appearance of SCVs. Imipenem and FQ resistance persisted for prolonged time once the selecting antimicrobial treatment had been discontinued. In contrast, resistance to -lactams rapidly disappeared after removal of the selective pressure, to reappear promptly upon renewed exposure. The results suggest that resistant P. aeruginosa are selected in less than 10 days independently of the antimicrobial class. Different resistance mechanisms lead to loss or persistence of resistance after removal of the selecting agent. Even if resistant isolates are not evident on culture, these may persist in the lung and can rapidly be reselected. Antimicrob. Agents Chemother. 2007 January 29; [Epub ahead of print] PMID: 17261619 [PubMed – as supplied by publisher]
also address possible mechanisms of resistance and pharmacokinetic limitations of the drug. The isolates underwent PFGE and exposure to phenyl-arginine--naphthylamide (PAbetaN), an efflux pump inhibitor. Patient 1 died of overwhelming A. baumannii infection and Patient 2 recovered after a change in antibiotic therapy. The MICs of tigecycline were 4 and 16 g/ml, respectively. Both isolates had a multidrug-resistant (MDR) phenotype and were genotypically unrelated. After exposure to PAbetaN, the MICs reduced to 1 and 4 g/ml, respectively. This is the first clinical description of bloodstream infection caused by tigecycline-non-susceptible A. baumannii. Such resistance appears to be at least partly attributable to an efflux pump mechanism. The authors urge caution when using tigecycline for treatment of A. baumannii bloodstream infection, given the reported low serum levels of this drug. J. Antimicrob. Chemother. 2007 January;59(1):128–31. [Epub 2006 November 1]. PMID: 17082201 [PubMed – indexed for MEDLINE] High tigecycline resistance in multidrug-resistant A. baumannii Carmeli and colleagues (Sackler Faculty of Medicine, Tel Aviv University, Israel) evaluated the in vitro activity of tigecycline against 82 non-replicate clinical isolates of MDR A. baumannii collected before the introduction of tigecycline. The majority (66%) of the isolates were resistant to tigecycline, 12% were intermediate and 22% were susceptible. The MIC50 and MIC90 of tigecycline were 16 and 32 g/ml, respectively, with a wide MIC range of 1–128 g/ml. The authors report high resistance rates to tigecycline, and higher than previously described MICs, in multiple clones of MDR A. baumannii. Their findings have disturbing clinical implications, as tigecycline represents a new treatment choice for infections caused by A. baumannii. J. Antimicrob. Chemother. 2007 March 12; [Epub ahead of print] PMID: 17353223 [PubMed – as supplied by publisher]
A. baumannii bloodstream infection while receiving tigecycline: a cautionary report Tigecycline has shown in vitro activity against A. baumannii, though published clinical experience with tigecycline outside clinical trials is lacking. A. baumannii is an opportunistic, often multiresistant gram-negative bacillus of increasing medical importance that can cause healthcareassociated infections and can survive for prolonged periods in the environment and on the hands of healthcare workers. Paterson and collegues (University of Pittsburgh Medical Center, Pittsburgh, PA, USA) describe bloodstream infections caused by tigecycline-non-susceptible A. baumannii in two patients receiving tigecycline for other indications. They
Multidrug-resistant Acinetobacter infection mortality rate and length of hospitalization The proportion of healthcare-associated infections caused by Acinetobacter spp. has increased over the past decade in the United States and worldwide. Sunenshine and colleagues (Centers for Disease Control and Prevention, Atlanta, Georgia, USA) performed a retrospective, matched cohort investigation at two Baltimore hospitals to examine outcomes of patients with MDR Acinetobacter infection compared with patients with susceptible Acinetobacter infections and patients without Acinetobacter infections. Multivariable analysis controlling for severity of illness and
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underlying disease identified an independent association between patients with MDR Acinetobacter infection (n = 96) and increased hospital and intensive care unit length of stay compared with 91 patients with susceptible Acinetobacter infection and 89 uninfected patients. Increased hospitalization associated with MDR Acinetobacter infection emphasizes the need for infection control strategies to prevent cross-transmission in healthcare settings. Emerg. Infect. Dis. 2007;13(1)2007, Posted 02/16/2007 Differentiation and distribution of colistin- and sodium dodecyl sulfate-tolerant cells in P. aeruginosa biofilms During P. aeruginosa flow cell biofilm development, the cell population differentiates into a nonmotile subpopulation which forms microcolonies and a migrating subpopulation which eventually colonizes the top of the microcolonies, resulting in the development of mushroom-shaped multicellular structures. Haagensen and colleagues (The Technical University of Denmark, Lyngby, Denmark) found that the cap-forming subpopulation developed tolerance to membrane-targeting antimicrobial agents, such as the cyclic cationic peptide colistin and the detergent sodium dodecyl sulfate. The stalk-forming subpopulation, on the other hand, was sensitive to the membrane-targeting antibacterial agents. All biofilm-associated cells were sensitive to the antibacterial agents when tested in standard plate assays. A mutation eliminating the production of type IV pili, and hence surface-associated motility, prevented the formation of regular mushroom-shaped structures in the flow cell biofilms; the development of tolerance to the antimicrobial agents was found to be affected as well. Mutations in genes interfering with lipopolysaccharide modification (pmr) eliminated the biofilm-associated colistin tolerance phenotype. Experiments with a PAO1 strain harboring a pmr–gfp fusion showed that only the cap-forming subpopulation in biofilms treated with colistin expresses the pmr operon. The authors suggest that increased antibiotic tolerance in biofilms is a consequence of differentiation into distinct subpopulations with different phenotypic properties. J. Bacteriol. 2007 January; 189(1):28–37. [Epub 2006 October 13] PMID: 17041046 [PubMed – indexed for MEDLINE]
2. Fungal infections Evaluating prophylaxis of invasive fungal infections in patients with haematologic malignancies Patients with hematologic malignancies are at substantial risk of developing invasive fungal infections (IFI) that
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are associated with substantial morbidity and mortality. Maertens (University Hospital Gasthuisberg, Leuven, Belgium) reviews the epidemiology, risk factors and efficacy of antifungal prophylaxis in patients with hematologic malignancies. The epidemiology of IFI has changed substantially in recent years with Candida albicans becoming less common owing to the widespread prophylactic use of azole antifungals. Invasive aspergillosis, fusariosis, and zygomycosis have increased in frequency. This change is at least partly related to the use of broad-spectrum antifungal agents, either as prophylaxis or as empirical treatment. Other risk factors for IFI include prior fungal exposure, immunosuppression, underlying disease, graft-versus-host disease, and organ dysfunction. Inconsistent results have been reported in studies evaluating the efficacy of antifungal prophylaxis in patients at risk of IFI. Meta-analyses found that antifungals, such as fluconazole and itraconazole, are effective in decreasing IFI and IFI-related mortality, primarily owing to yeast infections in patients with more severe immunosuppression (i.e. patients undergoing bone marrow transplantation), but do not decrease the overall mortality. The European Conference on Infections in Leukemia (ECIL) guidelines currently recommend fluconazole (AI, ie. strongly recommended, based on at least 1 well-executed, randomized trial) and itraconazole (BI, ie. generally recommended, based on at least one well-executed, randomized trial) in allogeneic transplant recipients. Posaconazole, a triazole antifungal, has been recently shown to decrease IFI incidence and overall mortality in some high-risk patients compared with standard azoles. Based on preliminary data, a provisional AI ECIL recommendation has been given. Eur. J. Haematol. 2007 January 23; [Epub ahead of print] PMID: 17241370 [PubMed – as supplied by publisher] The concentration-dependent nature of in vitro amphotericin B-itraconazole interaction against Aspergillus fumigatus: isobolographic and response surface analysis of complex pharmacodynamic interactions Meletiadis and colleagues (National Cancer Institute, Pediatric Oncology Branch, Bethesda, MD, USA) explored the in vitro combination of amphotericin B with itraconazole against 14 clinical isolates of Aspergillus fumigatus (9 itraconazole susceptible and 5 itraconazole resistant) with a colorimetric broth microdilution checkerboard technique using two drug interaction models able to explore complicated patterns of interactions: the response surface analysis of Bliss independence and the isobolographic analysis of Loewe additivity zero interaction theories. Synergy was found at combinations with low concentrations of amphotericin B (<0.125 g/ml), whereas antagonism was found at combinations with higher concentrations of amphotericin B. For itraconazole-resistant isolates, synergistic interactions were observed at high concentrations of
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itraconazole (>0.5 g/ml). Synergy was more frequently observed for the itraconazole-resistant isolates than for the itraconazole-susceptible isolates. Int. J. Antimicrob. Agents. 2006 November;28(5):439–49. [Epub 2006 October 19]. PMID: 17055706 [PubMed – indexed for MEDLINE] Multi-azole resistance in Aspergillus fumigatus Howard and colleagues (The University of Manchester, Manchester, UK) report multi-azole resistance in Aspergillus fumigatus from a patient who received long-term treatment with itraconazole and voriconazole for bilateral chronic cavitary aspergillosis with aspergillomas and whose isolates of A. fumigatus developed simultaneous resistance to itraconazole and voriconazole. A novel mutation (G138C) in the target gene (cyp51A) encoding 14␣-demethylase was detected. The patient had some response to intravenous caspofungin, which he received six times weekly, without the development of resistance over 9 months. Int. J. Antimicrob. Agents. 2006 November;28(5):450–3. [Epub 2006 October 10]. PMID: 17034993 [PubMed – indexed for MEDLINE] Use of fluconazole as a surrogate marker to predict susceptibility and resistance to voriconazole among 13,338 clinical isolates of Candida spp Voriconazole is a new triazole antifungal with an expanded spectrum and potency against Candida spp., Aspergillus spp., and other opportunistic fungal pathogens. Pfaller and colleagues (University of Iowa College of Medicine, Iowa City, IA, USA) carried out a study to addresses the use of fluconazole as a surrogate marker to predict the susceptibility of Candida spp. to voriconazole. Reference broth microdilution MIC results for 13,338 strains of Candida spp. isolated from more than 200 medical centers worldwide were used. Voriconazole MICs and interpretive categories (susceptible, ≤1 g/ml; susceptible dose dependent, 2 g/ml; resistant, ≥4 g/ml) were compared with those of fluconazole by regression statistics and error rate bounding analyses. For all 13,338 isolates, the absolute categorical agreement was 92% (false susceptible or very major error [VME], 0.0%). Since voriconazole is 16- to 32-fold more potent than fluconazole, the performance of fluconazole as a surrogate marker for voriconazole susceptibility was improved by designating those isolates with fluconazole MICs of ≤32 g/ml as being susceptible to voriconazole, resulting in a categorical agreement of 97% with 0.1% VME. The authors recommend that clinical laboratories performing antifungal susceptibility testing of fluconazole against Candida spp. use these results as surrogate markers until commercial FDA-approved voriconazole susceptibility tests become available.
J. Clin. Microbiol. 2007 January;45(1):70–5. [Epub 2006 November 1]. PMID: 17079501 [PubMed – indexed for MEDLINE] Posaconazole for the treatment of azole-refractory oropharyngeal and esophageal candidiasis in subjects with HIV infection Skiest and colleagues (University of Texas Southwestern Medical Center, Dallas, Texas, USA) evaluated the efficacy and safety of oral posaconazole for human immunodeficiency virus (HIV)-infected patients with oropharyngeal candidiasis (OPC) and/or esophageal candidiasis (EC) who were clinically refractory to treatment with oral fluconazole or itraconazole. Subjects received either oral posaconazole (400 mg twice daily) for 3 days followed by oral posaconazole (400 mg once daily) for 25 days (regimen A; 103 patients) or oral posaconazole (400 mg twice daily) for 28 days (regimen B; 96 patients). The primary end point was cure or improvement after 28 days. Primary efficacy analyses were performed on the subset of treated subjects with refractory disease (e.g., baseline culture positive for fluconazole- or itraconazole-resistant Candida species or persistent or progressive clinical signs or symptoms consistent with treatment failure). Of the modified intent-to-treat population, 132 (75%) of 176 subjects achieved a clinical response to posaconazole treatment. Clinical response rates were similar between regimen A recipients (75%) and regimen B recipients (75%). Clinical responses occurred in 67 (73%) of 92 subjects with baseline isolates resistant to fluconazole, 49 (74%) of 66 subjects with baseline isolates resistant to itraconazole, and 42 (74%) of 57 subjects with isolates resistant to both. Clinical response was achieved in 32 (74%) of 43 subjects with endoscopically documented EC. The most common treatment-related adverse events were diarrhea (11%), neutropenia (7%), flatulence (6%), and nausea (6%). Eight subjects (4%) discontinued therapy as a result of a treatmentrelated adverse event. Clin. Infect. Dis. 2007 February 15;44(4):607–14. [Epub 2007 January 17] PMID: 17243069 [PubMed – indexed for MEDLINE]
3. Viral infections 3.1. Influenza virus Antiviral resistance and the control of pandemic influenza The response to the next influenza pandemic will likely include extensive use of antiviral drugs (mainly oseltamivir), combined with other transmission-reducing measures. Ani-
Literature highlights / Drug Resistance Updates 10 (2007) 68–75
mal and in vitro studies suggest that some strains of influenza may become resistant to oseltamivir while maintaining infectiousness (fitness). Use of antiviral agents on the scale anticipated for the control of pandemic influenza will create an unprecedented selective pressure for the emergence and spread of these strains. Nonetheless, antiviral resistance has received little attention when evaluating these plans. Lipsitch and colleagues (Harvard School of Public Health, Boston, Massachusetts; Massachusetts General Hospital, Boston, Massachusetts; and Emory University, Atlanta, Georgia, USA) designed and analyzed a deterministic compartmental model of the transmission of oseltamivir-sensitive and -resistant influenza infections during a pandemic. The model predicts that even if antiviral treatment or prophylaxis leads to the emergence of a transmissible resistant strain in as few as 1 in 50,000 treated persons and 1 in 500,000 prophylaxed persons, widespread use of antivirals may strongly promote the spread of resistant strains at the population level, leading to a prevalence of tens of percent by the end of a pandemic. On the other hand, even in cir-
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cumstances in which a resistant strain spreads widely, the use of antivirals may significantly delay and/or reduce the total size of the pandemic. If resistant strains carry some fitness cost, then, despite widespread emergence of resistance, antivirals could slow pandemic spread by months or more and buy time for vaccine development; this delay would be prolonged by nondrug control measures (such as social distancing) that reduce transmission, or use of a stockpiled suboptimal vaccine. Surprisingly, the model suggests that such nondrug control measures would increase the proportion of the epidemic caused by resistant strains. The authors conclude that the benefits of antiviral drug use to control an influenza pandemic may be reduced, although not completely offset, by drug resistance in the virus. They recommend that the risk of resistance should be considered in pandemic planning and monitored closely during a pandemic. PLoS Med. 2007 January 23;4(1):e15 [Epub ahead of print] PMID: 17253900 [PubMed – as supplied by publisher]