Detection and reporting of organisms producing extended-spectrum beta-lactamases: Survey of laboratories in Connecticut

Extended-spectrum beta-lactamases (ESBLs) are enzymes produced in some gram -negative bacilli that mediate resistance to extended-spectrum cephalospori ns and aztreonam. They are most common in Klebsiella spp. and Escherichia c oli but are present in a variety of Enterobacteriaceae. Resistance mediated by these enzymes can be difficult to detect depending on the antimicrobial agents tested. AmpC beta-lactamases are related to the chromosomal enzymes of Enterobacter and Citrobacter spp, and also mediate resistance to extend ed-spectrum cephalosporins and aztreonam in addition to cephamycins, such a s cefoxitin. Unlike ESBLs, however, AmpC beta-lactamases are not inhibited by clavulanic acid or other similar compounds. To assess the abilities of v arious antimicrobial susceptibility testing methods to detect ESBLs, we sen t three ESBL-producing organisms, one AmpC-producing organism, and a contro l strain that was susceptible to extended-spectrum cephalosporins to 38 lab oratories in Connecticut for testing. Eight (21.0%) of 38 Labs failed to de tect extended-spectrum cephalosporin or aztreonam resistance in any of the ESBL- or AmpC-producing isolates. Errors were encountered with both automat ed and disk diffusion methods. Conversely, seven (18.4%) labs categorized a t least some of the four resistant isolates as potential ESBL producers and reported the results with the extended-spectrum cephalosporins and aztreon am as resistant as suggested by current National Committee for Clinical Lab oratory Standards (NCCLS) guidelines. The percentage of laboratories that f ailed to detect resistance in the ESBL or AmpC isolates ranged from 23.7 to 31.6% depending on the type of enzyme present in the test organism, This s urvey suggests that many laboratories have difficulty detecting resistance in ESBL and AmpC-producing organisms and may be unaware of the NCCLS guidel ines on modifying susceptibility testing reports for ESBL-producing strains .