Use of an Isogenic Escherichia coli Panel To Design Tests for Discrimination of β-Lactamase Functional Groups of Enterobacteriaceae

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Abstract
A study was designed to determine if an isogenic panel of Escherichia coli strains containing many different β-lactamases could be used for the preliminary screening of a large number of β-lactam agents to identify which might be most useful in the development of a definitive test for specific β-lactamases found among the members of family Enterobacteriaceae . The susceptibilities of 46 strains, comprising the isogenic panel, to expanded-spectrum cephalosporins, cephamycins, and aztreonam were determined in the presence and absence of β-lactamase inhibitors in broth microdilution tests. The results indicated that strains producing extended-spectrum β-lactamases (ESBLs) could be distinguished from strains producing other Bush-Jacoby-Medeiros functional group 2 or group 1 β-lactamases. For strains producing group 1 β-lactamases, cefpodoxime and ceftazidime MICs were ≥4 μg/ml and addition of clavulanate did not reduce the MICs more than fourfold. For strains producing group 2 enzymes other than ESBLs, cefpodoxime and ceftazidime MICs were ≤2 μg/ml. With a single exception (ceftazidime for the strain producing SHV-3), among strains producing ESBLs, cefpodoxime and ceftazidime MICs were ≥4 μg/ml and addition of clavulanate reduced the MICs by more than eightfold. Cephamycins could also be used to discriminate between strains producing group 1 β-lactamases and ESBLs, since only the former required cefotetan concentrations as high as 8 μg/ml or cefoxitin concentrations of >16 μg/ml for inhibition. Other cephalosporins provided some discrimination between the various β-lactamase producers, although they were not as reliable as either cefpodoxime or ceftazidime. These results indicate the utility of an isogenic panel for identification of candidate drugs among many for further testing with clinical isolates of the family Enterobacteriaceae to determine the best agents for detection of specific β-lactamases in this family.

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