DETECTION OF EXTENDED-SPECTRUM BETA-LACTAMASE (ESBL)-PRODUCING STRAINS BY THE ETEST ESBL SCREEN

Resistance to contemporary broad-spectrum beta-lactams, mediated by ex tended-spectrum beta-lactamase (ESBL) enzymes, is an increasing proble m worldwide. The Etest (AB Biodisk, Solna, Sweden) ESBL screen uses st able gradient technology to evaluate the MIC of ceftazidime alone comp ared with the MIC of ceftazidime with clavulanic acid (2 mu g/ml) to f acilitate the recognition of strains expressing inhibitable enzymes, I n the present study, ESBL-producing strains (17 Escherichia coli trans conjugants) were studied to define ''sensitive'' interpretive criteria for the Etest ESBL screen, These criteria (reduction of the ceftazidi me MIC by >2 log(2) dilution steps in the presence of clavulanic acid) defined a group of 92 probable ESBL-positive organisms among the 225 tested strains of Klebsiella species and E. coli having suspicious ant ibiogram phenotypes. With a subset of 82 clinical strains, the Etest E SBL screen was more sensitive (100%) than the disk approximation test (87%) and was more convenient. The MICs of ciprofloxacin, gentamicin, and tobramycin at which 50% of isolates are inhibited were 16- to 128- fold higher (coresistance) for the ESBL screen-positive group of strai ns than for the ESBL screen-negative group of strains, Some strains fo r which cephalosporin MICs were elevated and which were Etest ESBL scr een negative were also cefoxitin resistant, i.e., consistent with a ch romosomally mediated AmpC resistance phenotype, The Etest ESBL screen test with the ceftazidime substrate appears to be a useful method for detecting or validating the presence of enteric bacilli potentially pr oducing this type of beta-lactamase.