Occurrence and Phenotypic Characteristics of Extended-Spectrum β-Lactamases among Members of the Family Enterobacteriaceae at the Tel-Aviv Medical Center (Israel) and Evaluation of Diagnostic Tests

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Abstract
We assessed the prevalence and phenotypic characteristics of extended-spectrum β-lactamase (ESBL) producers among cefuroxime-resistant (CXM-R) (MIC ≥ 32 μg/ml) members of the family Enterobacteriaceae in our institution. The 438 CXM-R clinical isolates obtained from nonurine sources among inpatients were screened. ESBL production was confirmed by disk diffusion assay using cefpodoxime (CPD), cefotaxime (CTX), and ceftazidime (CTZ) with and without clavulanate (CLAV). A difference of ≥5 mm in the size of the zone of inhibition in the presence of CLAV for at least one of the agents was considered representative of the ESBL phenotype: 186 isolates (42.5%) were confirmed as ESBL producers. The isolates tested and the rates of ESBL producers were as follows: Klebsiella spp. ( n = 81), 79%; Proteus spp. ( n = 58), 62%; Escherichia coli ( n = 64), 53%; Enterobacter spp. ( n = 69), 42%; Serratia spp. ( n = 70), 14%; Citrobacter spp. ( n = 25), 24%; Providencia spp. ( n = 21), 24%; Morganella spp. ( n = 41), 5%; and Kluyvera ( n = 3), 0%. The overall sensitivity of isolated ESBL confirmatory tests was 79% for CPD-CLAV, 66% for CTZ-CLAV, and 91% for CTX-CLAV. Sensitivities of CTZ-CLAV confirmatory tests for Klebsiella spp., Proteus spp., E. coli , and Enterobacter spp. were 84, 22, 76, and 62%, respectively, and those for CTX-CLAV were 95, 97, 94, and 83%, respectively. They were 90% for CPD-CLAV and CTZ-CLAV, 95% for CPD-CLAV and CTX-CLAV, and 100% for CTZ-CLAV and CTX-CLAV. ESBL production was highly prevalent among Enterobacteriaceae. Using resistance to CXM as an ESBL screening criterion is a suitable option in high-incidence areas where Klebsiella spp. are not the dominant ESBL producers. This screening criterion may simplify the screening test and improve its sensitivity, although at the price of testing more isolates. The CTX-CLAV combination confirmed ESBL producers better than the CTZ-CLAV combination, with sensitivity varying between species. Combined CTZ-CLAV and CTX-CLAV testing detected all these strains; CPD-CLAV provided no additional benefit.

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