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Laboratory detection of bacteria with extended-spectrum beta-lactamases
Extended-spectrum beta-lactamases (ESBLs) are plasmid-mediated enzymes that hydrolyse and confer resistance to second and third generation cephalosporins, eg, cefuroxime, cefotaxime, and ceftazidime – the most widely used injectable antibiotics in many hospitals.
Until around 2001/2002, most ESBL-producing bacterial isolates in the United Kingdom (UK) were Klebsiella species, often from specialist units. Their ESBLs were cephalosporin-hydrolysing mutants of TEM and SHV – the common ‘old’ plasmid mediated penicillinases of Enterobacteriaceae. Laboratory methodology, therefore, evolved to detect these TEM and SHV variants, which mostly confer obvious resistance to ceftazidime with variable resistance to cefotaxime.
On this basis, it was recommended that ceftazidime should be used to screen for ESBL-mediated cephalosporin resistance in Enterobacteriaceae, with further confirmatory tests done on isolates found to be resistant. Since 2000, however, bacteria producing a new class of ESBL, CTX-M types, have emerged in the UK. The majority of producer isolates to date are Escherichia coli, often from patients at the hospital/community interface (eg, urinary infections in outpatients with a history of recent hospitalisation), although some patients do not appear to have had contact with hospitals. Isolates producing CTX-M ESBLs show obvious resistance to cefotaxime, with variable resistance to ceftazidime. Thus, screening with ceftazidime alone will fail to detect many producers. In view of this change the recommendations for screening Enterobacteriaceae have now been amended. Full details are available at
<http://www.hpa.org.uk/srmd/div_nsi_armrl/ESBL_advice_June_2004.pdf>, but the minimum recommendations can be summarised as:
Enterobacteriaceae from infections in hospitalised patients
Test both cefotaxime and ceftazidime on first-line panel, or test cefpodoxime.
Perform ESBL confirmatory tests (see grey box below) on isolates found resistant to any of cefotaxime, ceftazidime, or cefpodoxime.
Enterobacteriaceae from community patients
Test cefpodoxime as an indicator on first-line panel (one possible first-line panel for community urinary tract infection specimens comprises cefpodoxime, nitrofurantoin, trimethoprim, a fluoroquinolone, and two out of cephalexin, coamoxiclav, and ampicillin/amoxycillin).
Perform ESBL confirmatory tests (below) on isolates found resistant to cefpodoxime.
Identification to genus/species level is highly desirable for the interpretation of resistance patterns and, as a minimum, should be undertaken on all isolates found resistant to cefotaxime, ceftazidime, or cefpodoxime in the above tests.
To confirm ESBL production in isolates found resistant to cefotaxime/ceftazidime or cefpodoxime: examine for synergy between cefpodoxime and clavulanate using combination discs (a 5-mm larger zone to the clavulanate-containing disc indicates the presence of an ESBL). For Enterobacter spp. and C. freundii use cefpirome/clavulanate combination discs.