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Home »Detabase » Lactabase » Beta Lactamase » Classification » Molecular » Class A

Class A Beta Lactamase

These classes include:

ptr Narrow Spectrum Beta lactamases
ptr ESBL (Extended Spectrum Beta Lactamase)

ESBL (Extended Spectrum Beta Lactamase):

ESBL

Extended-Spectrum Beta-Lactamases (ESBLs) are actually enzymes produced by certain types of bacteria, which renders the bacteria resistant to the antibiotics commonly used to treat them. ESBLs were first discovered in the mid-1980s. At the time they were mostly found in the Klebsiella species of bacteria, in hospital intensive care units. Until recently, few people were affected by these mutated bacteria and it didn't appear to be a major growing concern. That has changed, however. According to the British Health Protection Agency (HPA), a new class of ESBL (called CTX-M enzymes) has emerged, which are now being widely detected among E.Coli bacteria. These ESBL-producing E. Coli are resistant to penicillins and cephalosporins, and are becoming more frequent in urinary tract infections. ESBLs defined as β-lactamases that are capable of hydrolysing oxyimino-cephalosporins and which (unlike AmpC types) are inhibited by clavulanic acid in vitro Some Species in Which ESBLs Are Found:

ptr Klebsiella
ptr Escherichia coli
ptr Enterobacter
ptr Proteus
ptr Citrobacter
ptr Pseudomonas*

Major classes include:

ptr TEM
ptr SHV
ptr CTX-M
ptr OXA

TEM


ptr TEM-type ESBLs derivatives of TEM-1 and TEM-2
ptr Possible first TEM-ESBL isolated in Liverpool in 1982; Klebsiella oxytoca harboured a gene encoding resistance to ceftazidime (TEM-12)
ptr Well over 100 TEM-type β-lactamases have been described, of which the majority are ESBLs
ptr Amino acid substitutions that occur within the TEM enzyme occur at a limited number of positions
ptr Combinations of these amino acid changes results in subtle alterations in the ESBL phenotype, e.g. the ability to hydrolyse ceftazidime or cefotaxime
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 to beta-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

SHV

ptr To date, the majority of SHV-type derivatives possess ESBL phenotype ptr Majority found in Klebsiella pneumoniae ptr In 1983, a Klebsiella ozaenae isolate from Germany was discovered to possess a beta -lactamase that hydrolysed cefotaxime
ptr Differed from SHV-1 by replacement of glycine by serine at the 238 position
ptr This mutation alone accounted for its extended spectrum properties
ptr Designated as SHV-2 - later found in organisms in every inhabited continent within 15 years of its discovery
ptr Selection pressure from 3rd generation cephalosporins thought responsible
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.

CTX-M

ptr Fast growing - important group
ptr Preferentially hydrolyse, and confer resistance to cefotaxime
ptr Escape of chromosomal β-lactamase genes from Kluyvera spp (a bug of no clinical importance!)
ptr Having migrated to mobile DNA, CTX-M β-lactamases genes may evolve further - undergoing mutations that increase activity against ceftazidime
ptr The first CTX-M ESBL in the UK was found as recently as 2000, in a solitary isolate of K. oxytoca
ptr First outbreak, caused by K. pneumoniae producing the new enzyme CTX-M-26, was recorded in Birmingham in 2001
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. CTX-M-15 is currently (2006) the most widespread type in E. coli the UK and is widely prevalent in the community.

CTX-M

CTX-Minsilico


July 2004 : Media 'discovers' CTX-M

OXA

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.

Recognising ESBLs in the Laboratory

Choice of Indicator Cephalosporin
ptr TEM & SHV - obvious resistance to ceftazidime, variable to cefotaxime
ptr CTX-M - obvious resistance to cefotaxime, variable to ceftazidime
ptr All ESBLs - obvious resistance to cefpodoxime

Confirmatory Tests for ESBLs
ptr Double-disc tests
ptr Practical and cost effective approach for routine detection
ptr However, optimal disc separation varies with the strain and some producers may be missed
ptr Combination disc methods
ptr Compare zones of inhibition of ceph alone, and ceph plus clavulanate
ptr Inexpensive and do not require critical disc spacing
ptr Etest ESBL strips
ptr Accurate and precise but more expensive than combination discs

Treatment Choice?
ptr ESBL-producing organisms hydrolyse many ß-lactam antibiotics, so choice of treatment is much reduced!
ptr Plasmids bearing the genes encoding ESBLs frequently carry genes encoding resistance to aminoglycosides and trimethoprim
ptr Increasing reports of plasmid-encoded decrease in susceptibility to quinolones, frequently in association with cephalosporin resistance
ptr Multiple ESBLs may reduce the effectiveness of β-lactam/β-lactamase inhibitor combinations
ptr Cephamycins are stable to ESBLs but loss of outer membrane porins may lead to resistance
ptr Studies assessing clinical outcomes of 3GC-treated ESBL infections have produced mixed results
ptr Failure with ceftazidime does not preclude success with another 3GC (e.g. cefotaxime)
ptr Success may depend on the type of ESBL being expressed, e.g. TEM-10 being resistant to ceftazidime but not other 3GCs
ptr However, poor choices for the treatment of serious infections due to ESBL-producing organisms

ESBLS - Conclusion
ptr ESBLs have evolved greatly over the last 20 years, with CTX-M type ESBLs becoming an increasing problem in the UK
ptr Overuse of the cephalosporins in the hospital setting has most likely caused the spread of ESBLs
ptr There is a need for formal treatment guidelines to be developed
ptr Screening for ESBLs in microbiology laboratories should be routine
ptr Presence of ESBLs will be sure to create significant therapeutic problems in the future, with resistance spreading
ptr Infection-control measures and reduction in use of third-generation cephalosporins are critical for limiting ESBLs in institutions
ptr ESBLs have evolved greatly over the last 20 years, with CTX-M type ESBLs becoming an increasing problem in the UK
ptr Overuse of the cephalosporins in the hospital setting has most likely caused the spread of ESBLs
ptr There is a need for formal treatment guidelines to be developed
ptr Screening for ESBLs in microbiology laboratories should be routine
ptr Presence of ESBLs will be sure to create significant therapeutic problems in the future, with resistance spreading
ptr Infection-control measures and reduction in use of third-generation cephalosporins are critical for limiting ESBLs in institutions