title: Detection of extended-spectrum beta-lactamase (ESBL)-producing strains by the Etest ESBL screen.
abstract: Resistance to contemporary broad-spectrum beta-lactams, mediated by extended-spectrum beta-lactamase (ESBL) enzymes, is an increasing problem worldwide. The Etest (AB Biodisk, Solna, Sweden) ESBL screen uses stable gradient technology to evaluate the MIC of ceftazidime alone compared with the MIC of ceftazidime with clavulanic acid (2 micrograms/ml) to facilitate the recognition of strains expressing inhibitable enzymes. In the present study, ESBL-producing strains (17 Escherichia coli transconjugants) were studied to define "sensitive" interpretive criteria for the Etest ESBL screen. These criteria (reduction of the ceftazidime MIC by > 2 log2 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 antibiogram phenotypes. With a subset of 82 clinical strains, the Etest ESBL 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 strains than for the ESBL screen-negative group of strains. Some strains for which cephalosporin MICs were elevated and which were Etest ESBL screen negative were also cefoxitin resistant, i.e., consistent with a chromosomally 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 producing this type of beta-lactamase.
authors.affiliation: Department of Pathology, University of Iowa College of Medicine, Iowa City 52242, USA.
authors.forename: M G
authors.initials: MG
authors.lastname: Cormican
authors.forename: S A
authors.initials: SA
authors.lastname: Marshall
authors.forename: R N
authors.initials: RN
authors.lastname: Jones
mesh_terms: D000900:Anti-Bacterial Agents / Q000378:metabolism*; D002442:Ceftazidime / Q000378:metabolism; D019818:Clavulanic Acid; D002969:Clavulanic Acids / Q000378:metabolism; D004755:Enterobacteriaceae / Q000187:drug effects / Q000201:enzymology*; D004926:Escherichia coli / Q000187:drug effects / Q000201:enzymology; D007709:Klebsiella / Q000187:drug effects / Q000201:enzymology; D008826:Microbial Sensitivity Tests; D011933:Reagent Kits, Diagnostic; D014162:Transfection; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D002969:Clavulanic Acids; D011933:Reagent Kits, Diagnostic; D019818:Clavulanic Acid; D002442:Ceftazidime; D001618:beta-Lactamases