mechanisms of resistance to β-lactams in some common gram-negative bacteria causing nosocomial...

Review

10.1586/14787210.3.6.915 © 2005 Future Drugs Ltd ISSN 1478-7210 915www.future-drugs.com

Mechanisms of resistance to β-lactams in some common Gram-negative bacteria causing nosocomial infectionsCarlos José Suárez, Karen Lolans, Maria Virginia Villegas and John P Quinn†

†Author for correspondenceChicago Infectious Disease Research Institute, 1900 W. Polk Room 1258, Chicago, IL 60612, USATel.: + 1 312 864 4874Fax: + 1 312 864 [email protected]

KEYWORDS: bacterial resistance, Gram-negatives, mechanism of drug resistance, nosocomial drug resistance

Gram-negative bacilli remain major killers of hospitalized patients and continue to evolve new resistance mechanisms. This review describes the mechanisms of resistance to β-lactam antibiotics from those Gram-negative pathogens most often isolated from nosocomial infections.

Expert Rev. Anti Infect. Ther. 3(6), 915–922 (2005)

Antimicrobial resistance continues to evolveand presents serious challenges in the therapyof both nosocomial and community-acquiredinfections; 50–60% of the more thantwo million nosocomial infections in the USAeach year are caused by antimicrobial-resistantbacteria [1]. Statistics from the US Departmentof Health and Human Services show thatnosocomial infections contribute to more than77,000 deaths per year.

With Gram-negative organisms having aplethora of resistance mechanisms at theirdisposal, their evolving resistance is clinicallyworrisome. In this review, the authors focus onmechanisms of resistance to β-lactams includ-ing β-lactamases, porins and efflux pumpsfound in common Gram-negative pathogens.

General mechanisms of resistance to β-lactams in Gram-negative bacteriaβ-lactamasesThe β-lactamases are enzymes that hydrolyzeβ-lactam agents. They are ubiquitous in Gram-negative bacilli and are the major cause ofGram-negative bacterial resistance toβ-lactams. Under the classification system ofAmbler, β-lactamases are divided into fourmolecular classes based upon their amino acidsequences according to whether their hydro-lytic mechanisms are serine-based (class A, C

and D) or dependant on a metal ion, themetalloenzymes (class B) [2]. The genes for theseenzymes can be either chromosomally encodedor plasmid mediated. The latter pose a sig-nificant threat in controlling bacterial resistance,as plasmid-mediated β-lactamase genes arereadily transferable among bacteria, allowing aneffective and quick spread of resistance.

Hundreds of β-lactamases have beendescribed, but the following currently have thegreatest impact in the nosocomial setting.

AmpC-type β-lactamases

Characteristically, these enzymes (alsodesignated as molecular class C), hydrolyze oxy-imino-β-lactams (third-generation cephalo-sporins), aztreonam and β-lactam/β-lactamaseinhibitor combinations, but also penicillins,narrow-spectrum cephalosporins (first- andsecond-generation cephalosporins) andcephamycins (cefoxitin and cefotetan). Theyare chromosomally encoded in many Gram-negative bacteria [3], and a useful acronym torecall the bacterial genus and species harboringthese AmpC β-lactamases is SPICEM:S = Serratia spp.; P = Pseudomonas aeruginosa,Providencia stuartii and Providencia rettgeri; I =Indol-positive Proteus spp.; C = Citrobacter fre-undii; E = Enterobacter spp.; M = Morganellamorganii. These have also been found to be

CONTENTS

General mechanisms of resistance to β-lactams in Gram-negative bacteria

Mechanisms of resistance in some common Gram-negative pathogens involved in nosocomial infections

Expert commentary

Five-year view

Key issues

References

Affiliations

For reprint orders, please contact [email protected]

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Suárez, Lolans, Villegas & Quinn

916 Expert Rev. Anti Infect. Ther. 3(6), (2005)

plasmid mediated in some Enterobacteriaceae such as Klebsiellapneumoniae and Salmonella spp. [4,5], that naturally lackchromosomal AmpC-type enzymes.

Extended-spectrum β-lactamases

First detected in the early 1980s, extended-spectrum β-lactam-ases (ESBLs) have diverse geographic distributions, and confer aremarkably variable susceptibility to different antibiotics [6].ESBLs confer resistance to aztreonam and oxyimino-cephalo-sporins (third-generation cephalosporins), as well as to penicillinsand narrow-spectrum cephalosporins. Most ESBLs have arisenfrom point mutations in genes encoding TEM- and SHV-typeenzymes. However, ESBLs also include some OXA-typeenzymes, the CTX-M-type family and other rather uncommonenzymes, found mainly in P. aeruginosa, such as PER-1, PER-2,SFO-1 and the GES/IBC family. The CTX-M family is the mostcommon non-TEM, non-SHV ESBL, preferentially hydrolyzingcefotaxime over ceftazidime in most cases [3]. Whereas ESBL-producing organisms are important causes of nosocomial infec-tions, CTX-Ms have also been implicated in community-onsetdisease [7]. CTX-M-producing strains are a major concern inLatin America and Asia, although their geographic distributionnow extends throughout Europe, North America and Africa [8].All ESBLs identified thus far are plasmid mediated. Organismscarrying ESBLs are often resistant to other antibiotics such as theaminoglycosides (tobramycin > gentamicin > amikacin), tetra-cyclines and trimethoprim/sulfamethoxazole as a result ofadditional resistance genes located on the plasmid harboring theESBL [9]. Coincidentally, many ESBL-producing strains are alsoresistant to quinolones, although the latter resistant determinantsare usually chromosomally encoded. ESBLs occur primarily inKlebsiella spp. and Escherichia coli, although they have beenreported in many Gram-negative species [3,10].

Carbapenemases

There are two main types of these enzymes, metallo-β-lactamases (MBLs; molecular class B) and serine carbapene-mases (molecular classes A and D). Currently, they are fairlyuncommon, although they are a source of considerable concerndue to a spectrum of activity that encompasses almost allknown β-lactams, from penicillins to the carbapenems (aztre-onam is spared for MBLs), and they are generally not susceptibleto inhibitors. The genes encoding these enzymes can be dividedinto those that are chromosomally mediated (frequentlyassociated with integrons) or on plasmids [11].

Serine carbapenemases have been sporadically described inenterobacterial isolates and Acinetobacter spp. In Entero-bacteriaceae, the few class A enzymes described include: NmcA,Sme-1 to -3, KPC-1 to -3, IMI-1 and GES-2, while fromAcinetobacter spp., a number of carbapenem-hydrolyzingoxacillinases (class D) have been identified (OXA 23–27,OXA-40 and OXA-58) [11].

MBLs mediate resistance in a wide range of Gram-negativeorganisms among which P. aeruginosa, and to a lesser extent,Serratia marcescens and Stenotrophomonas maltophilia, have the

greatest clinical impact. Naturally occurringchromosomal MBLs have been identified in a number ofGram-negative bacteria, such as S. maltophilia, Aeromonas spp.,Bacillus cereus and Flavobacterium spp., although most of theserarely cause serious infection. The two major MBL families,IMP and VIM, despite low amino acid homology (∼30%),nevertheless, have similar properties. They aretransferable MBLs, as most are found as gene cassettes locatedprimarily on class 1 integrons, sometimes within plasmids ortransposons. These enzymes are often associated with otherresistance gene cassettes, which commonly include aminoglyco-side-modifying enzymes, conferring resistance to multiple anti-biotic classes. Thus, they are clinically worrisome as a result oftheir broad spectrum of activity combined with the threat ofdissemination through these mobile genetic elements. Bothfamilies of MBLs are now widely spread in terms of hostspecies, although they are primarily found in P. aeruginosa andAcinetobacter baumannii. They are also widely distributed geo-graphically, having been detected in over 28 countries spanningfive continents [12]. Overall, MBLs mediate a higher level ofresistance than the serine carbapenemases, and are the subjectof intensive research at present [3,12].

Porin lossβ-lactams must traverse the outer membrane of Gram-negativebacilli through water-filled protein channels known as porins toreach their targets located in the inner membrane. The loss of aporin combined with production of a β-lactamase may conferresistance to one or many antibiotics simultaneously. Forinstance, a loss of a specific porin known as OprD in P. aerugi-nosa along with simultaneous production of AmpC confersresistance to carbapenems, particularly imipenem [9].

Efflux pumpsIn many Gram-negative bacilli, the bacteria may expel anti-biotics after entry by utilizing an energy-dependant mechanismknown as efflux [9]. Increasing arrays of efflux mechanisms havebeen identified in many clinically relevant Gram-negativeorganisms, including C. freundii, Enterobacter aerogenes, E. coli,K. pneumoniae, P. aeruginosa and S. maltophilia. These effluxpumps expel a vast array of molecules including metabolites,detergents, organic solvents, as well as clinically relevant anti-microbials, and their role in antimicrobial resistance has beenincreasingly appreciated [14]. The best studied and describedefflux mechanisms are those in P. aeruginosa, where four multi-drug efflux pump systems have been well characterized(MexAB-OprM, MexCD-OprJ, MexEF-OprN and MexXY-OprM); each having a preferential set of antimicrobialsubstrates [14,15]. These multidrug efflux determinants them-selves can mediate resistance to multiple classes of clinicallyrelevant antimicrobials: β-lactams (including meropenem andertapenem), fluoroquinolones, macrolides, aminoglycosidesand tetracyclines [16], and combined with porin mutationsand/or β-lactamase production, can result in resistance inP. aeruginosa to an even greater number of antibiotics [9].

Resistance mechanisms in Gram-negative bacteria

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Target modificationsA modification in the target to which the antibiotic binds isanother mechanism of bacterial resistance. For example, acommon mechanism of quinolone resistance in E. coli andP. aeruginosa involves mutations in the genes for DNA gyrase(topoisomerase II) or topoisomerase IV, leading to a reducedaffinity for the antibiotic [17,18]. Likewise, penicillin-bindingproteins (PBPs) are bacterial enzymes involved in cell wallsynthesis, and all β-lactams inhibit bacterial replication byinterfering with these targets. Alterations of these PBPs lead todiminished affinity for the antibiotic and may confer resistance.PBP mutations play an important role in much Gram-positivebacterial resistance, but typically are not importantdeterminants of resistance in Gram-negative bacilli [19].

Mechanisms of resistance in some common Gram-negative pathogens involved in nosocomial infectionsResistance among Escherichia coli & Klebsiella spp.The primary concern for emerging resistance among the entericGram-negative bacilli has been the decline in susceptibility forthe fluoroquinolones and the third-generation cephalosporins.The latter are due mostly to plasmid-mediated ESBLs.

The average prevalence of ESBLs by phenotype for the5 years of the SENTRY Antimicrobial Surveillance Program(1997–2001) showed the highest occurrence in Latin America(45.5%), followed by eastern and southern Europe (24.4%)and North America (6.9%) [20].

Most infections due to ESBL producers occur in the hospital;true community-acquired infections with an ESBL-producingstrain are uncommon. Risk factors for bacteremia with anESBL-producing strain include known colonization with anESBL-producing organism, residence in an intensive care unitor in a hospital area where ESBL producers are known to beendemic and recent use of a third-generation cephalosporin [21].

In centers known to harbor significant numbers of ESBL-producing strains, critically ill patients with nosocomialKlebsiella bacteremia should probably be treated with anti-biotics active against ESBL producers until the absence of anESBL is definitively established.

In ESBL-producing K. pneumoniae isolates causing bacteremia,carbapenems are the treatment of choice due to the lowermortality associated with the carbapenem over other antibioticoptions [22]. There is no published clinical experience withertapenem in the treatment of ESBL-producing K. pneumoniaebacteremia. However, if a K. pneumoniae isolate is susceptible toertapenem in vitro, there appears to be no reason that erta-penem would not be an acceptable choice for the treatment ofbacteremia, as it is active in vitro [23].

Resistance in Enterobacter spp.Enterobacter strains commonly become resistant tothird-generation cephalosporins and aztreonam due toderepression of its chromosomal AmpC β-lactamase. Undernormal circumstances, this inducible enzyme can be producedat low amounts, which does not significantly destroy the

third-generation cephalosporins. However, spontaneouslyoccurring mutations (rate 10-5 to 10-7) derepress the enzymeproduction, thereby generating enough enzyme to hydrolyzethese β-lactams [24]. Derepressed mutants can be selected frominitially susceptible bacterial populations during therapy withthird-generation cephalosporins. Once selected, these mutantscan accumulate in the hospital microflora. A recent large surveyof antibiotic resistance in intensive care units in the USArevealed that approximately 40% of Enterobacter spp. hadstably derepressed AmpC-mediated resistance [25].

A number of reports have emphasized the proclivity ofmembers of this genus to acquire broad β-lactam resistanceduring therapy with extended-spectrum cephalosporins. An illus-trative paper is the work of Chow and colleagues who reported129 cases of Enterobacter bacteremia at six medical centers in theUSA [26]. The mean age of infected patients was 59 years. Almostall patients had concomitant illnesses predisposing to Entero-bacter sepsis. For example, 42% of patients had undergone recentmajor surgery and 40% were mechanically ventilated. A total of29% of the strains were resistant to all β-lactams other thancarbapenems when first isolated in the laboratory. A total of 36out of 37 resistant isolates came from patients exposed to priorantibiotic therapy. In two-thirds of those cases, prior therapy hadincluded an extended-spectrum cephalosporin. This differencewas highly statistically significant compared with other agents.

Cefepime, cefpirome and carbapenems are stable to AmpCβ-lactamases [27]; therefore, owing to the risk of selectingderepressed mutants, these antibiotics should be the first-linetreatment for Enterobacter spp. infections in the majority ofclinical scenarios [28,29].

In recent years, AmpC genes have mobilized from parentEnterobacter and Citrobacter spp. onto plasmids, and thentransferred into E. coli or Klebsiella spp. This is typically due toa mobile genetic element such as a transposon, which can carrygenes from plasmid to chromosome and vice versa [5]. Thedetection and reporting of these strains remain a challenge toclinical microbiology laboratories, but currently publishedClinical and Laboratory Standards Institute standards (formerlyNational Committee for Clinical Laboratory Standards) shouldcorrectly guide appropriate therapy, and almost all such strainsare susceptible to carbapenems in vitro.

Resistance in Pseudomonas aeruginosaP. aeruginosa remains the most prevalent highly resistantpathogen to contemporary antimicrobial agents. Gales andcolleagues noted that P. aeruginosa isolates appear to be increasingin frequency in all SENTRY program regions, and distinctdifferences exist in their frequencies by geographic region andsite of infection [30]. For example, in this study, multidrug-resistant (MDR) P. aeruginosa appears to be most prevalent inEurope and Latin America, with low rates for North Americaand the Asia–Pacific regions.

In the hospital environment in North America, P. aeruginosacauses 3.9–4.6% of bacteremias, 18.1–18.7% of pneumoniasand 12.2% of overall infections in the intensive care unit.



P. aeruginosa is often resistant to a variety of broad-spectrumantimicrobial agents. In addition, initially susceptible strainsmay acquire drug resistance during treatment as a result of avariety of mechanisms. This has been reported with virtually allclasses of drugs including β-lactams, aminoglycosides andquinolones, as outlined in TABLE 1 [9].β-lactam resistance in P. aeruginosa is usually mediated by over-

production of a chromosomally mediated AmpC-type β-lactamaseresulting from mutations in regulatory genes. This results in clini-cally significant resistance to all third-generation cephalosporins.The fourth-generation cephalosporins, cefepime and cefpirome,remain somewhat more active than the third-generationcompounds under these circumstances, primarily due to theirhigher outer membrane permeability, lower affinity for β-lactama-ses and higher avidity for PBPs. However, strains of P. aeruginosamay acquire clinically significant resistance even to these agents bya combination of outer membrane impermeability and hyperproduction of an AmpC-type β-lactamase [9]. For the carbapen-ems, imipenem and meropenem, a major resistance mechanism isloss of the specific porin OprD [9]. Resistance has been shown tooccur in up to 50% of patients treated with imipenem for morethan 1 week [31]. Studies of isolates overexpressing OprD show thatthis porin is relatively specific for carbapenems and does notmediate passage of other β-lactams and quinolones [9].

However, for P. aeruginosa to develop resistance to mero-penem, loss of OprD is not sufficient. It has been known forsome time that the MexAB-OprM efflux system includesmost β-lactams in its spectrum, including meropenem [32,33].Based on this previous finding, Kohler and colleagues exam-ined the respective contributions of OprD and efflux systemson carbapenem resistance in P. aeruginosa, by constructingmutants with varying combinations of OprD and MexAB-OprM expression [34]. Their work demonstrated that mero-penem minimum inhibitory concentration (MIC) values werestrongly influenced by efflux, whereas imipenem remainedunaffected. Therefore, it appears that emergence of resistanceto meropenem occurs less commonly than to imipenem, asfull resistance to the former requires two separate mutations:loss of OprD plus overexpression of an efflux system activeagainst meropenem.

The high intrinsic antibiotic resistance that typifies P. aeru-ginosa has historically been attributed to impermeabilityacross the outer membrane. It has become increasingly clearthat this resistance is largely attributable to the activity ofseveral efflux pumps. Based on genomic data, at least fivedifferent pump families, encompassing many characterized aswell as predicted efflux pump systems, appear to bepresent [35].

Table 1. Mutational resistances in Pseudomonas aeruginosa.

Effect on strain, according to antipseudomonal drug

Mechanism Mutation site Fq Carb–Tic Pip–Azl Czid–Atm Cpm–Cpr Imi Mero Agl Pm

Reduced affinityTopoisomerase II gyrA r/R - - - - - - - -

Topoisomerase IV parC r/R - - - - - - - -

Derepression of AmpCPartial ampD - R R R r - - - -

Total ampD + other - R R R R - - - -

UpregulationMexAB-OprM nalB at mexR R/R R r/R r/R r/R - r - -

nalC at other

MexCD-OprJ nfxB r/R r/R r/R r/R R - r - -

MexEF-OprN nfxC at mexT r/R r/R r/R r/R r/R r r - -

MexXY-OprM r/R r/R r/R r/R r/R - r r/R -

Reduced aminoglycoside transport - - - - - - - r/R -

Loss of OprD oprD; nfxC at mexT - - - - - R r - -

Membrane changes - - - - - - - - R

Agl: Aminoglycosides; Atm: Aztreonam; Azl: Azlocillin; Carb: Carbenicillin; Cpm: Cefepime; Cpr: Cefpirome; Czid: Ceftazidime; Fq: Fluoroquinolone; Imi: Imipenem; Mero: Meropenem; Pip: Piperacillin; Pm: Polymyxin; r: Reduced susceptibility; R: Frank resistance, which may vary in its distinction from ‘r’: according to the breakpoints adopted; Tic: Ticarcillin.Reprinted with permission from D Livermore [9].



Quinolone-resistant strains of P. aeruginosa are relativelycommon. Mechanistically, these can be divided into twomajor classes, those resulting from target mutations in DNAgyrase and those resulting from efflux mutations.Quinolones may also select for mutants that are resistant toother classes of antibiotics and are known as multiple anti-biotic resistance mutants [36]. At least three related effluxmutations have been observed in the laboratory. These arenalB, nfxB and nfxC, all of which affect regulatory genesthat lead to overexpression of efflux pumps [34]. The nfxC(also known as mexT) mutation is particularly interestingbecause it leads to an overproduction of a membraneprotein, OprN, which is the outer membrane component ofan efflux system that pumps quinolones. These strainssimultaneously underexpress OprD. Therefore, thesemutants are cross resistant to quinolones, chloramphenicol,tetracycline and carbapenems [34].

Comprehensive resistance to the repertoire of useful anti-pseudomonal agents is made possible owing to an interplayof the aforementioned complex, yet efficient resistancemechanisms, and MDR clinical isolates are increasinglybeing reported [37,38]. Treatment options for MDRP. aeruginosa are limited in most cases, and combinationtherapy has long been considered the mainstay of therapeuticmanagement of these infections. Combinations of β-lactams,fluoroquinolones and aminoglycosides may achieve synergy,even when the strains are resistant to the individual anti-biotics [39,40]. Older drugs, like colistimethate, are beingrecognized and revived as useful alternatives in the treatmentof pan-resistant P. aeruginosa.

Resistance in AcinetobacterAcinetobacter are important nosocomial pathogens associatedwith serious infections that are trending toward increasing anti-microbial resistance [41,42]. Acinetobacter are of substantativeconcern because they are intrinsically resistant to a variety ofantimicrobial agents, including penicillins, cephalosporins andaminoglycosides. The broad-spectrum carbapenems, imipenemand meropenem, are effective against more than 90% of isolatesin the USA [43], but resistance to even these potent β-lactamshas been reported from centers worldwide [12,44].

The mechanisms underlying resistance to β-lactams have notbeen studied as thoroughly as those for P. aeruginosa, butsimilarly, resistance develops as a result of multiple mecha-nisms: β-lactamase (chromosomal- or plasmid-mediated)alterations in the PBPs or decreased membrane permeabilitydue to the loss of porins [44,45].

The chromosomal AmpC cephalosporinase confers resistanceto cephalosporins, and in combination with porin loss can conferlow-level resistance to the carbapenems [44]. Metalloenzymes ofthe IMP-type have been implicated in high-level carbapenemresistance in A. baumannii from Europe and Asia, although notfrom the Americas [12]. Class D OXA type enzymes alsomediate resistance to carbapenems and other β-lactams,although not as efficiently as the metalloenzymes [11].

Treatment options for serious MDR Acinetobacter, especiallywhen the efficacy of the carbapenem class has been compro-mised, are limited and as for P. aeruginosa, has led to a revival ofolder agents, such as colisthimethate.

Recently, a new antibiotic, tigecycline, with activity againstAcinetobacter was approved. In a study of 1087 bacterial strains,isolated in 12 medical centers located throughout Spain, theantimicrobial activity of tigecycline against 14 bacterial specieswas compared with that of other antimicrobial agents [46]. Thetigecycline MIC90 for all the tested Enterobacteriaceae isolatesranged from 0.5 to 2 µg/ml. After polymixin B, tigecycline wasthe most active agent against A. baumannii, with more than90% of strains inhibited by 8 µg/ml.

In the aforementioned study, sulbactam was the next agentwith significant intrinsic activity against A. baumannii. In aGerman study, 115 A. baumannii strains from a collection ofclinical isolates, obtained between 1991 and 2000 fromGermany, USA and various European countries were testedagainst ampicillin, piperacillin, amoxicillin, ticarcillin, clavu-lanic acid, tazobactam, sulbactam and combinations of theseβ-lactams with these β-lactamase inhibitors. Sulbactam alonewas more active against A. baumannii than any of these anti-biotics (MIC50: 2 µg/ml; MIC90: 32 µg/ml) [47]. Several studiescarried out worldwide have underscored its clinical efficacyagainst Acinetobacter [48,49].

Expert commentaryAntibiotic resistance to β-lactams among Gram-negative patho-gens in hospitals is a growing threat to our patients, especiallyin critical care units. The promiscuous abuse of antibiotics is acontributing factor, as exemplified by the rapid decline ofnumerous antibiotic classes against P. aeruginosa.

Inadequate attention to infection control measures is anothermodifiable variable. Surveys indicate that compliance withhandwashing is appalling when carefully scrutinized. The use ofalcohol-based disinfectants may be of some help here.

Many large pharmaceutical companies appear to haveabandoned the antibiotic-development battlefield, in manycases for business reasons [50]. In contrast to new drugsdeveloped for Gram-positive bacteria, there is a desperateneed for comparable development programs for multiresistantGram-negative organisms.

Five-year viewOur view is that the antibiotic pipeline for Gram-negativepathogens is very sparse. We do not see any development ofnew agents for multiresistant Gram-negatives on the horizon,therefore, for now, we must optimize therapy with older agents.

Conflict of interestDr Villegas has consulting arrangements and/or research grantsupport with BMS, Merck, Astra-Zeneca and Pfizer. Dr Quinnhas consulting arrangements and/or research grant support withAstra-Zeneca and Merck. None of these sources of support hascontributed to the preparation of this article in any way.



Key issues

• Antimicrobial resistance mechanisms in Gram-negative pathogens are constantly evolving.

• A complex interplay of several resistance mechanisms, such as the production of β-lactamases, loss of porins, efflux pumps and modifications to the target to which the antibiotic binds, account for the emerging resistance to β-lactams among Gram-negatives.

• The synthesis of β-lactamases is the predominant mechanism of resistance to β-lactams in Gram-negative bacteria.

• Plasmid-mediated β-lactamase genes are readily transferable among bacteria posing a significant threat to the control of bacterial resistance.

• At present, carbapenemases are rather uncommon, but they are a source of considerable concern as they hydrolyze virtually all classes of β-lactams and are spreading among bacteria.

• In Escherichia coli and Klebsiella pneumoniae, the main mechanism accounting for broad-spectrum resistance to β-lactams in the nosocomial setting is the production of extended-spectrum β-lactamases.

• Enterobacter possess a chromosomal, inducible AmpC-type β-lactamase, which confers broad-spectrum resistance to β-lactams when derepressed.

• In Pseudomonas aeruginosa, a combined loss of porins and overproduction of a chromosomally mediated AmpC-type β-lactamase confer resistance to fourth-generation cephalosporins, and even to imipenem if the porin lost is OprD.

• Full resistance to meropenem in P. aeruginosa requires two separate mutations: loss of OprD and overexpression of an efflux system active against meropenem.

• At present there are few therapeutic choices for multiresistant Gram-negatives. Therefore, there is an urgent need to develop new agents against these pathogens.

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Affiliations

• Carlos José Suárez, MD

Research Assistant, Bacterial Resistance Line, CIDEIM (International Center for Medical Research and Training), Avenue 1N No. 3–03, Cali, ColombiaTel.: + 57 2 668 2164Fax: + 57 2 667 [email protected]

• Karen LolansInfectious Disease Research Associate, Chicago Infectious Disease Research Institute, 1901 W. Harrison Street, Room LL715, Chicago, IL 60612, USATel.: + 1 312 864 7459Fax: + 1 312 864 [email protected]

• Maria Virginia Villegas, MD, MSc

Co-ordinator Bacterial Resistance Line, CIDEIM (International Center for Medical Research and Training), Avenue 1N No. 3–03, Cali, ColombiaTel.: + 57 2 668 2164Fax: + 57 2 667 [email protected]

• John P Quinn, MD

Scientific Director, Chicago Infectious Disease Research Institute, 1900 W. Polk Room 1258, Chicago, IL 60612, USATel.: + 1 312 864 4874Fax: + 1 312 864 [email protected]

mechanisms of resistance to β-lactams in some common gram-negative bacteria causing nosocomial...

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