Benefit and risk in the β-lactam antibiotic-resistance strategies of Streptococcus pneumoniae and Staphylococcus aureus

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19 Popham, D.L. and Sctlow, P. (1993)j. Bacterial. 175, 4870-4876 20 Tomb, J.F., El-Haji, H. and Smith, H.O. (1991) Gene 104, l-10 21 Asoh, S. et 01. (1986) Eur. 1. B~ochem. 160,231-238 22 Nakamura, hi. eta/. (1983) MO/. GUI. &net. 191, l-9 23 Zhang, Q.Y. and Spratt, B.G. (1989) Nucleic Acids Res. 17, 5383

24 Spratt, B.G. (1988) Nature 532, 173-176 25 Laible, G. et al. (1989) Mol. Microbial. 3, 1337-1348 26 Yanouri, A. et al. (1993) /. Bacterial. 175,7604-7616 27 Song, b1.D. et al. (1987) &BS Left. 221, 167-l 71 28 El Kharroubi, A. et al. (1991) Hiochem. 1. 2X0,463-469 29 Piras, G. et al. (1993)1. Bacterial. 17.5, 2844-2X.52 30 Dowson, CC;.. Hutchinson, A. and Spratt, B.G. (1989) Nucleic Acids Res. 17, 75 18 31 Gittins, J.K., Phoenix, D.A. and Pratt, J.lM. (1994) FEMS

Microbial. Rev. 13, l-12 32 (iranier, B. et al. (1992) Biocbem. /. 282, 78 l-788 33 .Llottl, H. et al. (1992) /. Bacferiol. 174, 3261-3269 34 Englcbcrt, S. et al. (1993) in Racterial Growth and Lpis (FEMS eds), Symp. 65) (De Pedro, M.A., HGltje, J.V. and Liiffelhatdt, ‘VI’., pp. 319-333, Plenum Publishers 35 Yousif, S.Y., Broome-Smith, J.K. and Spratt, B.G. (lY85)j. Gen. Microbivl. 131,2839-2845 36 Kcll, C.&I. et al. (1993) F/MS Microbial. /.ett. 106, 171-176 37 Cranier, B. et al. Methods Enzymol. (in press) 38 De Jonge, B. and Tomasz, A. (1993) Anfimicrob. Agents Chemother. 37, 342-346

39 Garcia-Bustos, J. and Tomasz, A. ( 1990) I’m. Nat/ Acad. Sci. USA 87, 541.5-5419 40 ‘I‘omasz, i\. (1994) Trends M~crob~ol.2, 380-3X.5 41 van tleijenoort, Y. et al. (1992) /. Bactenol. 174, 3549-3557 Biochemrstry. 42 blatsuhashi, ht. (1994) in New Comprehenswe Bacterial CeI \Vu// (Vol. 27), pp. 55-71, Elscvier Science Publishers 43 Laible, G., Spratt, B.G. and f lakcnbcck, R. (1991) Mol. Microbial. 5, 1993-2002 44 Gomez, M.J. ef al. (1993) in Bacterial Growth and Lysrs (EMS Symp. hT) (De Pedro, M.A., Hiiltje, J.V. and Ltiffclhardt, W.. eds), pp. 309-3 18, Plenum Publishers

4.5 Fraipont, (1. et al. (1994) Biochejlz./. 298, 189-195 46 Wu, F..et al. (1994) /. Bacterial. 176,443-449 47 Strynadka, N. et al. (1994) Nature 368, 657-660 Biochenzrstry. 48 Ayala, J.A. et al. (1994)in New Cornprehensive Bacterial Cell \Va/l (Vol. 27), pp. 73-109, Elsevier Science Publishers 49 Donachic, W.D. (1993) m Bacterial Growth and Lysis (FEMS Symp. 6.T) (De Pedro, !vl.A., Hijltje, J.V. and Lijffelhardt, W., eds), pp. 409-4 18, Plenum Publishers 50 Vinella, D., D’Ari, Ii. and Bouloc, I’. (1992) EMBO 1. 11, 1493-1501 51 Vinclla, D. and D’Ari, R. (1994) 1. Bacterial. 176. 956-972 52 Buchanan, C.E., Henriques, A.0. and Piggot, P.J. (19Y4) in New Comprehensive Biochemistry. Hacterial Cell Wall (Vol. 27), pp. 167-l 86, Elsevier Science Publishers 9, 435-442 53 Wang, Y. and Lutkenhaus, J. (1993) Mol. Microhwl.

Benefit and risk in the p-lactam antibiotic-resistance strategies of Streptococcus pneumoniae and StaDhvlococcusaureus L

Alexander Tomasz

S

anism are sluggish in growth The radically altered cell-wall chemistry in ince the late 198Os, the and unstable, suggesting that emergence and global penicillin-resistant pneumococci and the the modification of antibiotic stringent requirement for specific cell-wall spread of multidrug-resisttargets may exact an incapaciprecursors for expression of high-level ant pathogens that apparently tating price from the bacteria. retain their ability to cause liferesistance to methicillin in staphylococci Frequently, penicillin-resistant threatening disease has caught may represent compensating mechanisms laboratory isolates of pncumothe world by surprise. Two of to balance molecular risks produced by cocci and staphylococci in the strategies for antibiotic resistance the most recent and most sucwhich resistance is based on in these bacteria. cessful mechanisms of antialterations of the penicillinbiotic resistance to emerge A. Tonzasz is in thr l.aboratory of Mwobiology, binding proteins (PBPs) are resistance to penicillin in StrepThe Rockefeller University, 12.10 York Avenue, and unstable. It would also be tococcus pneumoniae New York, NY 10021, USA. expected that the genetic load resistance to methicillin in of multidrug resistance would Staphylococcus aureus (see impair full virulence and thus limit the pathogenic p. 421 for a glossary of antibiotics) - each involve potential (and clinical impact) of such bacteria. modification of the molecular targets of the antibiotics Clearly, acquisition of antibiotic resistance must in the bacterial cell. This is surprising because laborainvolve not only benefits, but also risks, for pathogenic tory mutants with this type of drug-resistance mech0 1994 EltevicrScience Lrd 0966 842.X/Y4/So7.00

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bacteria, which have to survive and multiply in the competitive and hostile environment of colonized or invaded hosts. What kinds of compensating mechanisms, if any, accompany the acquisition of genes encoding drug resistance in these bacteria? In this articlc, I focus on this question as I describe some unusual properties of p-lactam-resistant pneumococci and staphylococci, two major pathogens that now have highly successful multidrug-resistant clones in the clinical environment.

/ I,

MECHANISMS

Hun14315

Hun2863

!

Hun524

Target alteration in penicillin-resistant pneumococci All p-lactam antibiotics are acylating agents that bind to the active site(s) of PRPs. The superb and sclcctive inhibitory power of P-lactam compounds is r&ted to the structural Hun663 mimicry between the P-lactam ring and the carboxyl terminus (I)-alanyl-o-Ala residue) of the normal muropeptidc substrate of PBPs’. In contrast to other mechanisms of drug resistance, such as inactivating the antibiotic or genHun659 Hun146 erating permeability barriers to the drug, mechanisms that involve modification of the target cnzymcs always invoke the dilemma of how to fine-tune structural alterations so that they do not jeopardize the physiological function of the enzyme. _I_ ._._11 L,. -IA -1. Penicillin-resistant clinical isolates of 0 20 40 20 40 60 80 0. 60 80 pneumococci have modifications in four of the Time (min) five penicillin-sensitive PBPs that reduce their Fig. 1. HPLC analysis of pneumococcal cell-wall stem peptides. The elution profiles affinities towards p-lactam antibiotics* (see of cell-wall component stem peptides from six penicillin-resistant Hungarian strains also the review by Christopher Dow~on, of pneumococci (Hun) doffer both from the profile of a penicrllin-resistant South Tracey Coffey and Brian Spratt in this issue”). African strain (SA8249) and from that of a penicillin-susceptrble strain (R36A). The Does the structural alteration that reduces the peptide peaks identified by numbers are major components, the structures of which are given in Refs 4.5. affinity for penicillin (substrate analog) also affect the physiological function of these pncumococcal proteins in cell-wall synthesis during normal growth in a drug-free medium? Careful together with resistance to penicillin in multistep examination of growth rates and other properties of transformation experiments, suggesting that the cellpenicillin-resistant pneumococci has shown no eviwall alterations may be a consequence of altcrcd dence for physiological impairment, indicating that functioning of the remodeled I’HPs in the penicillinthe bacteria have found some way to circumvent the resistant bacteria’. For example, the altered active site risky aspects of their mechanism of drug resistance”. (or some other domain in the remodeled PBPs) may Insight into a possible such compensating mechanism give rise to an altered substrate preference for these comes from analysis of the cell-wall peptidoglycan of enzymes. In this model, the branched muropeptidcs penicillin-resistant isolates. in the pcptidoglycan of penicillin-resistant pneumococci represent cell-wall precursors that are structurally a better ‘fit’ into the altered active site (or Cell-wall alterations Purified cell walls of penicillin-resistant South some other relevant site) of the remodeled PHPs than African (and former Czechoslovakian) strains of arc the normal linear peptides. This may enable the remodeled PBPs to catalyst! cell-wall synthesis pneumococci contain cell-wall peptidoglycan with a radically altered composition compared with that of effectively when penicillin is absent from the growth medium. penicillin-susceptible strains (Fig. 1). There appears Thus, a critical feature of antibiotic resistance in to be a nearly quantitative replacement of linear stem penicillin-resistant pneumococci may also involve the peptides (the major building blocks of peptidoglycan ability of these bacteria to produce an adequate in penicillin-susceptible pneumococci) with branched supply of ‘altered’ cell-wall substrates that are tailored stem peptidcs in which the E-amino group of the to fit the altered active sites of the penicillin-resistant Iysine is substituted by short dipcptides (alanyl-Ala or PBPs. Most interestingly, the chemical abnormalities in seryl-Ala) (Fig. 2)4. This abnormal feature of the pcpthe cell walls of the penicillin-resistant isolates appear tidoglycan can bc transferred, to a substantial degree,

I

.1

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MECHANISMS

Fig. 2. Substitution of the e-amino group of lysine residues with dipeptides in the cell walls of penicillin-resistant (South African clone X) pneumococci (b) compared with penicillin-susceptible pneumococci (a). G, N-acetylglucosamine; M, N-acetylmuramic acid; iGlu, isoglutamine.

to be specific to the clone, ivhich closely parallels the clone-specific polymorphism of PBP-encoding genes and PBPs that occurs in these bacteria’. Target alteration in methicillin-resistant S. aureus In contrast to penicillin-resistant pncumococci, mcthicillin-resistant S. aweus retain a normal set of (highaffinity) PBPs. The low-affinity target that enables thcsc bacteria to grow in the presence of high con-

centrations of virtually all p-&tam antibiotics is the product of the mecA gene - a 2 130 bp stretch of nonstaphylococcal (‘foreign’) DNA prcscnt in all mcthicillin-resistant isolates (the origin of mecA is reviewed by Gordon Archer and Debra Niemeyer in this issueh) which is a 78 kDa protein (PBP2a or PBP2’) with extremely low affinity for all p-lactam antibiotics’. In the absence of antibiotics, cell-wall synthesis in these bacteria is generally assumed to be catalysed by the normal set of PBPs, and PBP2a may only begin to function as an alternative enzyme when the normal PBPs are inactivated by antibiotic. Thus, the mechanism of resistance to p-lactams used by staphylococci seems to avoid the molecular dilemma faced by the remodclcd pneumococcal PBPs: how to perform normal cell-wall synthesis with a set of abnormal (low-affinity) proteins? However, this drug-resistance strategy may lead to several different types of risks for the bacteria. In susceptible staphylococci, acylation of PBPs rapidly leads to cell death and autolysis. Since drug-resistant staphylococci retain the normal set of high-affinity PBPs, what prevents the killing and lysis of the cells in the presence of the antibiotic? It must be assumed that drugresistant staphylococci contain some as-yct-unidcntified mechanism in addition to PBP2a (antibiotic tolerant??) that blocks the potentially fatal consequcnccs of encounter with p-lactams. Another potential risk may be related to the extraspecies origin of mecA. Because PBP2a, the key component of the staphylococcal p-lactam-resistance strategy,

RUSAIIM

is a non-native

protein,

might the effective functioning of this protein in the ‘foreign’ environment of a staphylococcus impose some special physio011 the logical requirements cell? Kecent genetic analysis and high-resolution biochemical techniques have produced interesting observations and provocative that may be related to this aspect of the drug-resistance strategy of staphylococci.

Str&ent substrate requiwments synthesis

for cell-wall

150

0

Time (min) Fig. 3. HPLC elution profiles of cell-wall muropeptides generated from a highly methicillin-resistant (parental) Staphylococcus aureus (COL) and the altered muropeptide proflles of Tn551-inactivated auxiliary mutants. Numbers and letters refer to the chemical structure of the muropeptides, as described In Ref. 12. (Reproduced with permission from Ref. 12.)

It has been known for some time that intact mecA and its product PBP2a alone cannot explain fully the methicillin-resistant phenoall mcthicillintype because resistant S. aweus isolates, whatever their minimal inhibitory (MICs) (from concentrations as low as 3 mgl-’ to as high as 1600 mgl-‘), contain comparable amounts of PBP2a (Refs 9,lO). Such major disparities bctwecn cellular amounts of PBP2a and the MIC for the antibiotic suggest that other, unknown factor(s) (‘factor X’) are

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MECHANISMS

& + Z-Z

Muropeptlde pathway

t=

G-v ?‘a

Bactoprenol o

IQIU &?Io,,, Slgnalllng (effector) tunctlons In membrane AhnC%T?lal (e.g. turn off PBP2a)

\YS - (Gly), 4 (D)?la (o) Ala

-

Competltlon (wlth B_lactams) for active site and/or regulatory sltes on PBPs lweffsctive

1 Incorporation Into wall Structural defect Fig. 4. Alternative models to explain the mechanism(s) by which the level of resistance to methicillin is reduced [minimal inhibitory concentration (MIC) is low] in auxiliary mutants (blue) compared with strains with normal auxiliary functions (and a high MIC for methicillin) (black). PBP. penicillin-binding protein. For other abbreviations, see the legend to Fig. 2.

also essential in the phenotypic expression of resistance’. This idea also came out of early genetic studies in which the level of resistance to mcthicillin could be reduced by inscrtional inactivation of a chromosomal site ((12003) within the so-called femA gene (factor essential for the expression of resistance to methicillin), which is outside the mecA dcterminant”. Recently, several additional chromosomal sites outside mecA have been identified in which transposon inactivation reduces the lcvcl of resistance to fi-lactamsh~‘z. The importance of these genes becomes clear when it is considered that it is the appropriate functioning of thcsc determinants (in the genetic background) rather than the quantity of PBP2a in the cells that seems to determine the ,MIC value of an isolate’.“‘. It is not yet clear how many such ‘auxiliary genes’ exist and exactly how these genes cooperate with the mecA gene to bring about high-level resistance to p-lactams (see the review by Brigitte Berger-Bachi in this issue”). Nevertheless, some clues are emerging from recent biochemical studies that have shown that several of the mutants in auxiliary genes have an altered cell-wall composition (Fig. 3). Tn.<_51 inactivation of any one of at least three chromosomal determinants needed to complete the pentaglycine crossbridges on the E-amino group of the staphylococcal muropeptides leads both to the production of chemically abnormal pcptidoglycan and to a drastic reduction in the MIC for methicillin’4~‘5. Similarly, inactivation of a fourth gene that seems to control the optimal rate of production of the pcnta-

peptide cell-wall precursor reduces the level of resistance to methicillin 1 OO-fold”. Transposon insertion into a gene that controls amidation of the a-carboxyl groups in the stem peptide results both in peptidoglycan with an abnormal muropeptide composition and in a dramatic drop in the level of resistance16. Why do these interfercnccs with the synthesis of ‘normal’ muropeptides (that is, the inactivation of auxiliary genes) cause such a striking reduction in the phenotypic expression of resistance to /3-lactams in methicillin-resistant S. auretls, despite the presence of large amounts of PBP2a? There is, so far, no definitive answer to this question. One model’* assumes that the effective functioning of PBP2a also requires an abundant supply of structurally correct cell-wall building blocks that then compete successfully with mcthicillin for the active site of PBP2a. Alternatively, muropeptides of ‘incorrect’ structure (for example, with fewer than five glycine units in the crosslinking peptides, as in femB mutants, or with deficient amidation of the glutamic acid residues, as in the mutant RUSA208) may not be able to perform some as-yet-unidentified effector function. Abnormal precursor stem peptidcs that arc incorporated into the cell wall may cream localized structural defects. All or any of these factors would lead to a decrease in the MIC for methicillin (Fig. 4).

Virulence and antibiotic resistance Drug-resistant bacterial pathogens, such as penicillin-resistant pncumococci and methicillin-resistant S. atlretis, that emerge in the clinical environment must

MOI,ECUI_AR

MECHANISMS

to bc prcscnt may bc an incope with two kinds of selective creased ability to adhere to host pressures: an antibiotic-saturated cells at colonization sites. environment and also the more ‘traditional’ selection for the Geographical spread of a presence of at least a minimal m&drug-resistant clone of number of gcnctic traits essential methicillin-resistant S. aurcus for bacterial survival in their My laboratory has recently natural environment of the colonanalysed several hospital outized and/or invaded host. A carcbreaks of discasc due to mcthiful, quantitative assessment of the cillin-resistant staphylococci in presence and expression of viruseveral hospitals in Spain and lence-related properties in peni,Mcthicillin-resistPortugal”,22. cillin-resistant clones of pneumoFig. 5. The similarity of genetic backant staphylococci wcrc not reand methicillin-resistant cocci grounds in penicillin-resistant capsular ported in Spain until early 1989, staphylococci has not yet been type 66 pneumococcal isolates from when three outbreaks occurred Nevertheless, completed. the Iceland and Spain. The figure shows in Barcelona, IMadrid and molecular epidemiological evipulsed-field gel electrophoretic patterns of some Icelandic and Spanish isolates which were soon Valencia, dence for the remarkable ability after Smal restriction of chromosomal followed by 24 more outbreaks of multidrug-resistant pncumoDNA (reproduced with permission from all over Spain”. A detailed cocci and staphylococci for exRef. 18). analysis of the outbreak at the tensive geographical spread, two Princeps d’Espanya-Bellvitge examples of which are described Hospital using molecular-epidemiology techniques below, suggests that carrying resistance genes and has identified a single clone that was responsible virulcncc genes may be compatible, at least in some for ~-85% of disease (and colonization) caused by clones of these bacterial pathogens. all wards methicillin-resistant S. aureus throughout of the hospital. Most interestingly, the same clone Geographical spread of a multidrug-resistant was identified in Madrid, Lisbon and Porto, over capsular type 6R S. pneumoniae There were no detectable penicillin-resistant isolates of 1000 km from Barcelona”. It is not clear what feature S. pneumoniae in Iceland during 1983-1988. The first of this particular clone leads to its dominance and continued prevalence in the hospital environment. penicillin-resistant pncumococcus was isolated in December 1988 and, during the next 3 years, the freOne conceivable virulence-related property in this quency of penicillin-resistant isolates rose steeply from clone may be an increased ability to adhere to colonization and infection sites and/or to intravenous 2.3% (1989), 2.7% (1990) and 8.4% (1991) to 17% catheters. of all pneumococcal isolates in the first quarter of 1992l;. A surprisingly large fraction (almost 70%) of the isolates had a serotype B capsule and had virtually Conclusion identical multidrug-resistant phenotypes, including Clearly, hosting a multitude of antibiotic-resistance resistance to penicillin, tetracycline, chloramphenicol, gcncs (most of them ‘borrowed’ from foreign sources) erythromycin and trimethoprim-sulfamethoxazole. and retaining virulence-related propcrtics are well My laboratory has examined a sample (57 isolates) of within the adaptive capacity of pneumococci and these drug-resistant strains using the methods of molstaphylococci. A better understanding of how these ecular epidemiology. All 57 isolates (except one) had foreign-born genes are integrated into the metabolism a common multilocus enzyme type, pulsed-field gel of these bacteria should provide fascinating insights electrophoretic pattern (after SmaI digestion of the into novel mechanisms of gene expression and may chromosomal DNA) and a unique PBP pattern, also identify novel targets for antibiotic development which wcrc the same as the corresponding patterns against the drug-resistant pneumococcus and staphyloseen in penicillin-resistant serotype 6B pncumococci coccus. from Spainix (Fig. 5). While some geographically unique penicillin-resistant pneumococcal clones have Acknowledgement Work described hcrc has received partial support through a grant from been reported previously’g~20, the abrupt appearance the Sational Institutes of Health (K01 A116794), US Public Health of a single multidrug-resistant pneumococcal clone Service. with a high incidence in a geographically confined area such as Iceland is quite unprecedented. It seems References likely that the Icelandic strains originated in Spain, 1 Chuyscn, J-M. (1991) Annrc. Rev. Mioobiol. 45, 37-67 either directly or by some indirect route. 2 Zighelboim, S. and Tomasz, .A. (1980) Antirnicrob. Agents I’he ability of the multidrug-resistant pneumococci Chemother.17,434-442 to cause human disease (primarily otitis media and 3 Dowson, C.G., Coffey, T.1. and Spratt, B.C. (1994) Twds infections of the upper respiratory tract) shows that Microhiol. 2, 361-366 antibiotic resistance and genetic traits essential for 4 Garcia-Rustos, J.F., Chair, R. and ‘I‘omnsz, r\. (1988)j. Uucted. some aspects of pncumococcal virulence have become IiO, 2143-2147 associated in this particular clone. A trait that is likely 5 Garcia-Bustos, j. and Tomasz, A. (1YYO)PYOC. Xrltl kad. SC;.

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USA 87,5414-5419 6 Archer, CL. and Xicmeyer, D&l. (1994) 7’rmfs Mwobio/. 2, 343-347 7 Song, M.D. et al. (1987) FEBS I,ett. 221, 167-171 8 Handwerger, S. and Tomasz, A. (19X.5)Rev. Infect. Dis. 7, 368-386 9 Hartman, B.J. and Tomasz, A. (1986) Antmicrob. Agerzts Chetnother. 29, 85-92 10 Murakami, K. and Tomasl, A. [ 1989) /. Hacterid. 171, 874-879 11 Bergcr-Bachi, B. (1983) 1. Bacterial. 154,479-487 12 DC Lcncastrc, H. et 01. (1994) ,/. Antinzicroh. Chemother. 33, 7-24 13 Berger-BCchi, 1’1.(1YY4)Trerzds hiicrohiol. 2. 389-393 14 De jongc, B.L.M. et nl. (I 992) _/.Bid. Chem. 267,

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112.5.511259 1.5 Maidhof, H. et nl. (19Yl)j. Racteriol. 173, 3507-3513 16 Ornelas-Soares, A. ef al. (1993) 1. Hid. Chem. 268, 26268-26272 17 Kristinsson, K.G., Hjalmarsdottir, bJ.A. and Stcingrimsson, 0. (1992) Lanret 339,1606-1607 18 Soarcs, S. et nl. (lY93)j. Irzfect. Dis. 168, 1.58-163 19 Munoz, R. eta/. (1992) Clin. Infect. Dis. IS, 112-l 18 20 Linares, J. et al. (1992) C/in. Inf~t. Dis. 1.i, 99-105 21 DC Lcncastrc, H. et nl. (1994) Eur. ]. C/in. Microhiol. Infect. Dis. 13,64-73 22 Dominguez, MA. et al. (1994) 1. Clirz. !IJicrobiol.32,

2081-2087 23 Pujol, M. et al. ( 1994)Eur. _/. C/IV. Mwubiol. Infect. Dis. 13, 96-l 02

Bacterial resistance to the cyclic glycopeptides David M. Shlaes and Louis B. Rice

T

conscrvcd, and some of these he cyclic glycopcptidcs Cyclic-glycopeptide antibiotics, such as vancomycin and teicoplanin, have been conserved amino acids are arc antibiotics that arc almost uniformly active against pathogenic highly active against crucial to the mode of action Gram-positive bacteria since their Gram-positive bacteria. Vancoof these antibiotics. Other immycin, which is widely used in discovery in the 19.50s. Resistance is portant components include the (JSA, is gcncrally more active now emerging among cntcrococci and chlorine substituents and staphylococci by acquisition of novel against the staphylococci, while sugars’“. The cyclic glycopepteicoplanin, which is currently genes or by mutation, respectively. tides bind to the terminal two The mechanism of resistance for used in Europe, but has not yet residues (acyl-r>-alanyl-o-Ala) been released in the USA, is more cnterococci appears to be synthesis of of the pentapeptide peptidoactive against enterococci. Other an altered ceil-wall precursor with glycan precursor” as it is cyclic glycopeptides, including lower affinity for the antibiotics. exported across the cell memristocetin and actaplanin, have brane to the cell wall. The D.M. Shims and L.B. Rice are itz the Research not been used to treat human binding of n-Ala-n-Ala by the Seruice mzd lnfectlous Diseasrs Section, Dept of infections’. (See p. 421 for a bulky antibiotic is thought to Vcteratzs Affairs Medical Center, Dept of Medicine, glossary of antibiotics.) block interaction of the transCnsc Westwn Resarue Uuiuersity, ClcLvland, Entcrococci with varying glycosylase with the pcptidolevels of resistance to vancoglycan, and thus prevent mycin and teicoplanin have been reported worldgrowth of the cell-wall peptidoglycan (Fig. 2). Almost wide2-h. Some of these strains have caused serious inall bacteria synthesize peptidoglycan terminating in fections and many are resistant to both aminoglycosides D-Ala-u-Ala, but the exclusion limits of the porin and penicillins, making treatment very difficult. In the proteins of Gram-ncgativc outer membranes prcvcnt first 100 cases of infection with vancomycin-resistant transport of the glycopeptides, and so only Gramenterococci in New York, the mortality rate for bacpositive species are susceptible to clinically achievable teremia was about 25X, a figure that is similar to that concentrations of this class of antibiotics. quoted in large published reviews’,*. Epidemiological studies have described both clonal spread and infection Mechanisms of resistance to glycopeptldes with sporadic strains’. The spread of vancomycinThe vancomycin-resistant enterococci can be divided resistant entcrococci in the USA has been rapid (Fig. 1). into three phenotypic groups: A, B and C, the important characteristics of which are summarized in Table 1. The origin of the van genes in enterococci is not Mechanism of glycopeptide activity The cyclic glycopeptidcs have molecular masses of known, and a search of glycopeptide-producing bacterial strains with vnn-gene probes has been unreward1200-2000 Da. They all have a central core heptaing. The vanA gene has been cloned and sequenced by peptide containing three amino acids that are highly