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Beta-lactamase

    Beta-lactamase is a type of enzyme () produced by some bacteria that is responsible for their resistance to beta-lactam antibiotics like penicillins cephalosporins cephamycins and carbapenems These antibiotics have a common element in their molecular structure: a four-atom ring known as a beta-lactam The lactamase enzyme breaks that ring open deactivating the molecule's antibacterial properties
    Beta-lactamases produced by gram-positive organisms are usually secreted
    The structure of a Streptomyces β lactamase is given by

    Penicillinase

    Penicillinase is a particular type of β lactamase showing specificity for penicillins again by hydrolysing the beta-lactam ring Molecular weights of the various penicillinases tend to cluster near 50000
    Penicillinase was the first β lactamase to emerge due to the early dependence on penicillin Penicillinase-resistant β lactams were then developed such as methicillin although there is widespread resistance to these now also (see MRSA)

    Resistance in Gram Negative Bacteria

    Among gram-negative bacteria the emergence of resistance to expanded-spectrum cephalosporins has been a major concern It appeared initially in a limited number of bacterial species (E cloacae , C freundii S marcescens and P aeruginosa ) that could mutate to hyperproduce their chromosomal class C β-lactamase A few years later resistance appeared in bacterial species not naturally producing AmpC enzymes (K pneumoniae Salmonella spp P mirabilis) due to the production of TEM- or SHV-type ESBLs Characteristically such resistance has included oxyimino- (for example ceftizoxime cefotaxime , ceftriaxone and ceftazidime as well as the oxyimino-monobactam aztreonam )but not 7-alpha-methoxy-cephalosporins (cephamycins) or in other words (cefoxitin and cefotetan ), has been blocked by inhibitors such as clavulanate sulbactam or tazobactam and did not involve carbapenems Plasmid-mediated AmpC β-lactamases represent a new threat since they confer resistance to 7-alpha-methoxy-cephalosporins (cephamycins) such as cefoxitin or cefotetan are not affected by commercially available β-lactamase inhibitors and can in strains with loss of outer membrane porins provide resistance to carbapenems *Philippon A, Arlet B, Jacoby GA2002 "Plasmid-determined AmpC-type β-lactamases" Antimicrob Agents Chemother 2002; 46:1-11 hey

    Extended-Spectrum Beta-Lactamases (ESBL)

    Members of the family Enterobacteriaceae commonly express plasmid-encoded b-lactamases (eg TEM-1 TEM-2 and SHV-1) which confer resistance to penicillins but not to expanded-spectrum cephalosporins . In the mid-1980s a new group of enzymes the extended-spectrum b-lactamases (ESBLs) was detected(first detected in germany in 1983)*Knothe H, Shah P. Kremery V, et al 1983 "Transferable resistance to cefotaxime cefoxitin cefamandole and cefuroxime in clinical isolates of Klebsiella pneumoniae and Serratia marcescens" Infection 1983; 11: 315-7 ESBLs are beta-lactamases that hydrolyze extended-spectrum cephalosporins with an oxyimino side chain These cephalosporins include cefotaxime ceftriaxone and ceftazidime as well as the oxyimino-monobactam aztreonam ESBLs confer resistance to expanded-spectrum cephalosporins (eg ceftriaxone cefotaximeand ceftazidime ), aztreonam and related oxyimino-beta lactams Typically they derive from genes for TEM-1 TEM-2 or SHV-1 by mutations that alter the amino acid configuration around the active site of these ß-lactamases This extends the spectrum of ß-lactam antibiotics susceptible to hydrolysis by these enzymes An increasing number of ESBLs not of TEM or SHV lineage have recently been described*Emery C. L., and L. A. Weymouth 1997 "Detection and clinical significance of extended-spectrum ß-lactamases in a tertiary-care medical center" J Clin Microbiol 35:2061-2067 . The ESBLs are frequently plasmid encoded Plasmids responsible for ESBL production frequently carry genes encoding resistance to other drug classes (for example aminoglycosides) Therefore antibiotic options in the treatment of ESBL-producing organisms are extremely limited Carbapenems are the treatment of choice for serious infections due to ESBL-producing organisms yet carbapenem-resistant isolates have recently been reported ESBL-producing organisms may appear susceptible to some extended-spectrum cephalosporins However treatment with such antibiotics has been associated with high failure rates

    ESBL Types

    *Bradford PA2001 "Extended-spectrum β-lactamases in the 21st century:characterization epidemiology and detection of this important resistance threat" Clin Microbiol Rev 2001; 48:933-51*George A. Jacoby MD and Luisa Silvia Munoz-Price MD2005 "mechanisms of : The New beta-Lactamases" N Engl J Med 2005;352:380-91

    TEM beta-lactamases (class A)

    TEM-1 is the most commonly encountered beta-lactamase in gram-negative bacteria Up to 90% of ampicillin resistance in E coli is due to the production of TEM-1 Also responsible for the ampicillin and penicillin resistance that is seen in H influenzae and N gonorrhoeae in increasing numbers Although TEM-type beta-lactamases are most often found in E coli and K pneumoniae they are also found in other species of gram-negative bacteria with increasing frequency The amino acid substitutions responsible for the ESBL phenotype cluster around the active site of the enzyme and change its configuration allowing access to oxyimino-beta-lactam substrates Opening the active site tob-lactam substrates also typically enhances the susceptibility of the enzyme to b-lactamase inhibitors such as clavulanic acid Single amino acid substitutions at positions 104 164 238 and 240 produce the ESBL phenotype but ESBLs with the broadest spectrum usually have more than a single amino acid substitution Based upon different combinations of changes currently 140 TEM-type enzymes have been described TEM-10 TEM-12 and TEM-26 are among the most common in the United States*Paterson DL, Hujer KM, Hujer AM, et al2003 [http://aacasmorg/cgi/reprint/47/11/3554 "Extended-spectrum b-lactamases in Klebsiella pneumoniae bloodstream isolates from sevencountries: dominance and widespread prevalence of SHV- and CTX-M-type b-lactamases"] Antimicrob Agents Chemother 2003;47:3554-60

    SHV beta-lactamases (class A)

    SHV-1 shares 68 percent of its amino acids with TEM-1 and has a similar overall structure The SHV-1 beta-lactamase is most commonly found in
    K pneumoniae and is responsible for up to 20% of the plasmid-mediated ampicillin resistance in this species ESBLs in this family also have amino acid changes around the active site most commonly at positions 238 or 238 and 240 More than 60 SHV varieties are known They are the predominant ESBL type in Europe and the United States and are found worldwide SHV-5 and SHV-12 are among the most common*Paterson DL, Hujer KM, Hujer AM, et al2003 [http://aacasmorg/cgi/reprint/47/11/3554 "Extended-spectrum b-lactamases in Klebsiella pneumoniae bloodstream isolates from sevencountries: dominance and widespread prevalence of SHV- and CTX-M-type b-lactamases"] Antimicrob Agents Chemother 2003;47:3554-60

    CTX-M beta-lactamases (class A)

    These enzymes were named for their greater activity against cefotaxime than other oxyimino-beta-lactam substrates (eg ceftazidime ceftriaxone or cefepime ). Rather than arising by mutation they represent examples of plasmid acquisition of beta-lactamase genes normally found on the chromosome of Kluyvera species a group of rarely pathogenic commensal organisms These enzymes are not very closely related to TEM or SHV beta-lactamases in that they show only approximately 40% identity with these two commonly isolated beta-lactamases More than 40 CTX-M enzymes are currently known Despite their name a few are more active on ceftazidime than cefotaxime They have mainly been found in strains of Salmonella enterica serovar Typhimurium and E coli but have also been described in other species of Enterobacteriaceae and are the predominant ESBL type in parts of South America(They are also seen in eastern europe) CTX-M-14 CTX-M-3 and CTX-M-2 are the most widespread

    OXA beta-lactamases (class D))

    OXA beta-lactamases were long recognized as a less common but also plasmid-mediated beta-lactamase variety that could hydrolyze oxacillin and related anti-staphylococcal penicillins These beta-lactamases differ from the TEM and SHV enzymes in that they belong to molecular class D and functional group 2d . The OXA-type beta-lactamases confer resistance to ampicillin and cephalothin and are characterized by their high hydrolytic activity against oxacillin and cloxacillin and the fact that they are poorly inhibited by clavulanic acid Amino acid substitutions in OXA enzymes can also give the ESBL phenotype While most ESBLs have been found in E coli K pneumoniae and other Enterobacteriaceae the OXA-type ESBLs have been found mainly in P aeruginosa OXA-type ESBLs have been found mainly in Pseudomonas aeruginosa isolates from Turkey and France The OXA beta-lactamase family was originally created as a phenotypic rather than a genotypic group for a few beta-lactamases that had a specific hydrolysis profile Therefore there is as little as 20% sequence homology among some of the members of this family However recent additions to this family show some degree of homology to one or more of the existing members of the OXA beta-lactamase family Some confer resistance predominantly to ceftazidime but OXA-17 confers greater resistance to cefotaxime and cefepime than it does resistance to ceftazidime

    Others

    Other plasmid-mediated ESBLs such as PER VEB GES and IBC beta-lactamases have been described but are uncommon and have been found mainly in P aeruginosa and at a limited number of geographic sites PER-1 in isolates in Turkey France and Italy; VEB-1 and VEB-2 in strains from Southeast Asia; and GES-1 GES-2 and IBC-2 in isolates from South Africa France and Greece PER-1 is also common in multiresistant acinetobacter species in Korea and Turkey Some of these enzymes are found in Enterobacteriaceae as well whereas other uncommon ESBLs (such as BES-1 IBC-1 SFO-1 and TLA-1) have been found only in Enterobacteriaceae

    Inhibitor-Resistant β-Lactamases

    Although the inhibitor-resistant β-lactamases are not ESBLs they are often discussed with ESBLs because they are also derivatives of the classical TEM- or SHV-type enzymes These enzymes were at first given the designation IRT for inhibitor-resistant TEM β-lactamase; however all have subsequently been renamed with numerical TEM designations There are at least 19 distinct inhibitor-resistant TEM β-lactamases Inhibitor-resistant TEM β-lactamases have been found mainly in clinical isolates of E coli but also some strains of K pneumoniae Klebsiella oxytoca P mirabilis and Citrobacter freundii Although the inhibitor-resistant TEM variants are resistant to inhibition by clavulanic acid and sulbactam thereby showing clinical resistance to the beta-lactam--lactamase inhibitor combinations of amoxicillin-clavulanate (Co-amoxiclav) ticarcillin-clavulanate and ampicillin/sulbactam they remain susceptible to inhibition by tazobactam and subsequently the combination of piperacillin/tazobactam To date these beta-lactamases have primarily been detected in France and a few other locations within Europe*Bradford PA2001 "Extended-spectrum β-lactamases in the 21st century:characterization epidemiology and detection of this important resistance threat" Clin Microbiol Rev 2001; 48:933-51

    AmpC type β-lactamases (class C)

    They are another group of enzymes that are commonly isolated from extended-spectrum cephalosporin-resistant Gramnegative bacteria AmpC β-lactamases (also termed class C or group 1) are typically encoded on the chromosome of many Gramnegative bacteria including
    Escherichia coli Citrobacter freundii and Enterobacter spp but can also be found on plasmids*Philippon A, Arlet B, Jacoby GA2002 "Plasmid-determined AmpC-type β-lactamases" Antimicrob Agents Chemother 2002; 46:1-11 . AmpC β-lactamases in contrast to ESBLs hydrolyse broadand extended-spectrum cephalosporins (cephamycins as well as to oxyimino-blactams) but are not inhibited by clavulanic acid or other β-lactamase inhibitors

    Carbapenemases (classes A, B, and D)

    Carbapenems are famously stable to AmpC β-lactamases and extended-spectrum-β-lactamases Carbapenemases are a diverse group of b-lactamases that are active not only against the oxyimino-cephalosporins and cephamycins but also against the carbapenems Aztreonam is stable to the metallo-β-lactamasesbut many IMP and VIM producers are resistant owing to other mechanisms

    IMP-type carbapenemases(one of the metallo-b-lactamases)

    Plasmidmediated IMP-type carbapenemases 17 varieties of which are currently known became established in Japan in the 1990s in both enteric gram-negative organisms and in pseudomonas and acinetobacter species IMP enzymes spread slowly to other countries in the Far East were reported from Europe in 1997 and have been found in Canada and Brazil

    VIM (Verona integron-encoded metallo-b-lactamase)

    A second growing family of carbapenemases the VIM family was reported from Italy in 1999 and now includes 10 members which have a wide geographic distribution in Europe South America and the Far East and have been found in the United States VIM-1 was discovered in P. aeruginosa in Italy in 1996; subsequently VIM-2 -now the predominant variant- was found repeatedly in Europe and the Far East; VIM-3 and -4 are minor variants of VIM-2 and -1, respectively VIM enzymes mostly occur in P. aeruginosa also P. putidaand very rarely Enterobacteriaceae
    Amino acid sequence diversity is up to 10% in the VIM family 15% in the IMP family and 70% between VIM and IMP Enzymes of both the families nevertheless are similar1 Both are integron-associated sometimes within plasmids Both hydrolyse all β-lactams except monobactams and evade all β-lactam inhibitors

    OXA group of b-lactamases (Class D)

    mainly occurring in Acinetobacter species divided into two clusters OXA carbapenemases hydrolyse carbapenems very slowly in vitro and the high MICs seen for some Acinetobacter hosts (>64 mg/L) may reflect secondary mechanisms They are sometimes augmented in clinical isolates by additional resistance mechanisms such as impermeability or efflux

    KPC (K. pneumoniae carbapenemase) (Class A)

    A few class A enzymes notably the plasmid-mediated KPC enzymes are effective carbapenemases as well Three variants are known distinguished by one or two amino-acid substitutions KPC-1 was found in North Carolina KPC-2 in Baltimore and KPC-3 in New York They have only 45% homology with SME and NMC/IMI enzymes and unlike them can be encoded by self-transmissible plasmids

    SME IMI NMC and CcrA

    generally of little clinical significance
    CcrA (CfiA) Its gene occurs in c. 1-3% of B. fragilis isolates but fewer produce the enzyme since expression demands appropriate migration of an insertion sequence CcrA was known before imipenem was introduced and producers have shown little subsequent increase

    Treatment of ESBL/AmpC

    Generally an isolate is suspected to be an ESBL producer when it shows in vitro susceptibility to the second generation cephalosporins (cefoxitin cefotetan) but resistance to the third generation cephalosporins and to aztreonamMoreover one should suspectthese strains when treatment with these agents for Gram negative infections fails despite reported in vitro susceptibility Once an ESBL producing strain is detected the laboratory should report it as “resistant” to all penicillins cephalosporins and aztreonam evenif they test as susceptible Associated resistance to aminoglycosides and trimethoprim-sulfamethoxazole as well as high frequency of co-existence of fluoroquinolone resistance creates problems Beta-lactamase inhibitors such as clavulanate sulbactam or tazobactam in vitro inhibit most ESBLs but the clinical effectiveness of beta-lactam/beta-lactamase inhibitor combinations cannot be relied on consistently for therapy Although cephamycins (cefoxitin and cefotetan) not hydrolyzed by majority of ESBLs but are hydrolyzed by associated AmpC-type ß-lactamase similarly ß-lactam/ß-lactamase inhibitor combinations may not be effective against organisms that produce AmpC-type ß-lactamase Also sometimes these strains decrease the expression of outer membrane proteins rendering them resistant to cephamycins In vivo studies have yielded mixed results against ESBL-producing
    K pneumoniae (Cefepime a fourth-generation cephalosporin has demonstrated in vitro stability in the presence of many ESBL/AmpC strains) Currently carbapenems are generally regarded as the preferred agent for treatment of infections due to ESBL-producing organisms Carbapenems are resistant to ESBL-mediated hydrolysis and exhibit excellent in vitro activity against strains of Enterobacteriaceae expressing ESBLs
    For organisms producing TEM and SHVtype ESBLs apparent in vitro sensitivity to cefepime and to piperacillin–tazobactam is common butboth drugs show an inoculum effect with diminished susceptibility as the size of the inoculum is increased from 100000 to 10000000 organisms Strains with some CTX-M–type and OXA-type ESBLs are resistant to cefepime on testing despite the use of a standard inoculum Strains producing only ESBLs are susceptible to cephamycins and carbapenems in vitro and show little if any inoculum effect with these agents AmpC-producing strains are typically resistant to oxyimino-blactams and to cephamycins and are susceptible to carbapenems; however diminished porin expression can make such a strain carbapenem-resistant as well38 Strains with IMP- VIM- and OXA-type carbapenemases usually remain susceptible to aztreonamResistance to non–b-lactam antibiotics is common in strains making any of these enzymes such that alternative options for non–b-lactam therapy need to be determined by direct susceptibility testing Resistance to fluoroquinolones and aminoglycosides is especially high
    For infections caused by ESBL-producing E. coli or klebsiella species treatment with imipenem or meropenem has been associated with the best outcomes in terms of survival and bacteriologic clearance Cefepime and piperacillin–tazobactam have been less successful Ceftriaxone cefotaxime and ceftazidime have failed even more often despite the organism’s susceptibility to the antibiotic in vitro107 Several reports have documented failure of cephamycin therapy as a result of resistance due to porin loss108109 Some patients have respondedto aminoglycoside or quinolone therapy but in a recent comparison of ciprofloxacin and imipenem for bacteremia involving an ESBL-producingK pneumoniae imipenem produced the better outcome There have been few clinical studies to define the optimal therapy for infections caused by ESBLproducing P. aeruginosa strains

    Notes


    Reference

    • Jacoby GA, Munoz-Price LS. The new β-lactamases N Engl J Med 2005;352:380-91 PMID 15673804

    • http://wwwlaheyorg/studies/webtasp#OXA

    • http://wwwncbinlmnihgov/entrez/queryfcgi?db=pubmed&cmd=Retrieve&dopt=AbstractPlus&list_uids=12558458&query_hl=3&itool=pubmed_docsum

    See also

    • Beta-lactamase inhibitors