may also occur on escherichia coli but is not usually inducible, although it can be hyperexpressed. ampc type β-lactamases may also be carried on plasmids. ampc β-lactamases, in contrast to esbls, hydrolyse broad and extended-spectrum cephalosporins (cephamycins as well as to oxyimino-β-lactams) but are not inhibited by β-lactamase inhibitors such as clavulanic acid. ampc-type β-lactamase organisms are often clinically grouped through the acronym, "space": serratia, pseudomonas or proteus, acinetobacter, citrobacter, and enterobacter. carbapenemases carbapenems are famously stable to ampc β-lactamases and extended-spectrum-β-lactamases. carbapenemases are a diverse group of β-lactamases that are active not only against the oxyimino-cephalosporins and cephamycins but also against the carbapenems. aztreonam is stable to the metallo-β-lactamases, but many imp and vim producers are resistant, owing to other mechanisms. carbapenemases were formerly believed to derive only from classes a, b, and d, but a class c carbapenemase has been described. imp-type carbapenemases (metallo-β-lactamases) (class b) plasmid-mediated imp-type carbapenemases (imp stands for active-on-imipenem), 19 varieties of which are currently known, became established in japan in the 1990s both in 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-β-lactamase) (class b) 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; since then, 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. 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 similar. both are integron-associated, sometimes within plasmids. both hydrolyse all β-lactams except monobactams, and evade all β-lactam inhibitors. the vim enzymes are among the most widely distributed mbls, with >40 vim variants having been reported. biochemical and biophysical studies revealed that vim variants have only small variations in their kinetic parameters but substantial differences in their thermal stabilities and inhibition profiles. oxa (oxacillinase) group of β-lactamases (class d) the oxa group of β-lactamases occur mainly in acinetobacter species and are 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. oxa carbapenemases also tend to have a reduced hydrolytic efficiency towards penicillins and cephalosporins. kpc (k. pneumoniae carbapenemase) (class a) a few class a enzymes, most noted the plasmid-mediated kpc enzymes, are effective carbapenemases as well. ten variants, kpc-2 through kpc-11 are known, and they are distinguished by one or two amino acid substitutions (kpc-1 was re-sequenced in 2008 and found to be 100% homologous to published sequences of kpc-2). 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. as of february 2009, the class a klebsiella pneumoniae carbapenemase (kpc) globally has been the most common carbapenemase, and was first detected in 1996 in north carolina, usa. a 2010 publication indicated that kpc producing enterobacteriaceae were becoming common in the united states. cmy (class c) the first class c carbapenemase was described in 2006 and was isolated from a virulent strain of enterobacter aerogenes. it is carried on a plasmid, pymg-1, and is therefore transmissible to other bacterial strains. sme (serratia marcescens enzymes), imi (imipenem-hydrolysing β-lactamase), nmc and ccra in general, these are of little clinical significance. ccra (cfia). its gene occurs in ca. 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. ndm-1 (new delhi metallo-β-lactamase) (class b) main article: new delhi metallo-beta-lactamase originally described from new delhi in 2009, this gene is now widespread in escherichia coli and klebsiella pneumoniae from india and pakistan. as of mid-2010, ndm-1 carrying bacteria have been introduced to other countries (including the united states and uk), most probably due to the large number of tourists travelling the globe, who may have picked up the strain from the environment, as strains containing the ndm-1 gene have been found in environmental samples in india. ndm have several variants which share different properties. treatment of esbl/ampc/carbapenemases general overview in general, an isolate is suspected to be an esbl producer when it shows in vitro susceptibility to the cephamycins (cefoxitin, cefotetan) but resistance to the third-generation cephalosporins and to aztreonam. moreover, one should suspect these 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, even if it is tested (in vitro) 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, and tazobactam in vitro inhibit most esbls, but the clinical effectiveness of beta-lactam/beta-lactamase inhibitor combinations cannot be relied on consistently for therapy. cephamycins (cefoxitin and cefotetan) are not hydrolyzed by majority of esbls, but are hydrolyzed by associated ampc-type β-lactamase. also, β-lactam/β-lactamase inhibitor combinations may not be effective against organisms that produce ampc-type β-lactamase. 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, in general, 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. esbls strains producing only esbls are susceptible to cephamycins and carbapenems in vitro and show little if any inoculum effect with these agents. for organisms producing tem and shv type esbls, apparent in vitro sensitivity to cefepime and to piperacillin/tazobactam is common, but both drugs show an inoculum effect, with diminished susceptibility as the size of the inoculum is increased from 105 to 107 organisms. strains with some ctx-m–type and oxa-type esbls are resistant to cefepime on testing, despite the use of a standard inoculum. inhibitor-resistant β-lactamases although the inhibitor-resistant tem variants are resistant to inhibition by clavulanic acid and sulbactam, thereby showing clinical resistance to the beta-lactam—beta 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. ampc ampc-producing strains are typically resistant to oxyimino-beta lactams and to cephamycins and are susceptible to carbapenems; however, diminished porin expression can make such a strain carbapenem-resistant as well. carbapenemases strains with imp-, vim-, and oxa-type carbapenemases usually remain susceptible. resistance to non-beta-lactam antibiotics is common in strains making any of these enzymes, such that alternative options for non-beta-lactam therapy need to be determined by direct susceptibility testing. resistance to fluoroquinolones and aminoglycosides is especially high. escherichia coli or klebsiella for infections caused by esbl-producing escherichia 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 vitro. several reports have documented failure of cephamycin therapy as a result of resistance due to porin loss. some patients have responded to aminoglycoside or quinolone therapy, but, in a recent comparison of ciprofloxacin and imipenem for bacteremia involving an esbl-producing k. pneumoniae, imipenem produced the better outcome pseudomonas aeruginosa there have been few clinical studies to define the optimal therapy for infections caused by esbl producing pseudomonas aeruginosa strains. use as a pharmaceutical in 1957, amid concern about allergic reactions to penicillin-containing antibiotics, a beta-lactamase was sold as an antidote under the brand name neutrapen. it was theorized that the breakdown of penicillin by the enzyme would treat the allergic reaction. while it was not useful in acute anaphylactic shock, it showed positive results in cases of urticaria and joint pain suspected to be caused by penicillin allergy. its use was proposed in pediatric cases where penicillin allergy was discovered upon administration of the polio vaccine, which used penicillin as a preservative. however, some patients developed allergies to neutrapen. the albany hospital removed it from its formulary in 1960, only two years after adding it, citing lack of use. some researchers continued to use it in experiments on penicillin resistance as late as 1972. it was voluntarily withdrawn from the american market by 3m pharmaceuticals in 1997. detection beta-lactamase enzymatic activity can be detected using nitrocefin, a chromogenic cephalosporin substrate which changes color from yellow to red upon beta-lactamase mediated hydrolysis. evolution beta-lactamases are ancient bacterial enzymes. metallo β-lactamases ("class b") are all structurally similar to rnase z and may have evolved from it. of the three subclasses b1, b2, and b3, b1 and b2 are theorized to have evolved about one billion years ago, while b3 seems to have arisen independently, possibly before the divergence of the gram-positive and gram-negative eubacteria about two billion years ago. pngm-1 (papua new guinea metallo-β-lactamase-1) has both metallo-β-lactamase (mbl) and trnase z activities, suggesting that pngm-1 is thought to have evolved from a trnase z, and that the b3 mbl activity of pngm-1 is a promiscuous activity and subclass b3 mbls are thought to have evolved through pngm-1 activity. subclasses b1 and b3 has been further subdivided. serine beta-lactamases (classes a, c, and d) appear to have evolved from dd-transpeptidases, which are penicillin-binding proteins involved in cell wall biosynthesis, and as such are one of the main targets of beta-lactam antibiotics. these three classes show undetectable sequence similarity with each other, but can still be compared using structural homology. groups a and d are sister taxa and group c diverged before a and d. these serine-based enzymes, like the group b betalactamases, are of ancient origin and are theorized to have evolved about two billion years ago. the oxa group (in class d) in particular is theorized to have evolved on chromosomes and moved to plasmids on at least two separate occasions. etymology the "β" (beta) refers to the nitrogen's position on the second carbon in the ring. lactam is a blend of lactone (from the latin lactis, milk, since lactic acid was isolated from soured milk) and amide. the suffix -ase, indicating an enzyme, is derived from diastase (from the greek diastasis, "separation"), the first enzyme discovered in 1833 by payen and persoz.