Evaluation of phenotypic screening methods for detecting plasmid-mediated AmpC β-lactamases-producing isolates of Escherichia coli and Klebsiella pneumoniae

Detection of plasmid-mediated (P-M) AmpC β-lactamase-producing isolates is considered critical for epidemiologic studies and hospital infection control, but the documents of the Clinical and Laboratory Standards Institute do not contain any recommendation for the phenotypic detection. In this study, phenotypic detection methods, cefoxitin-Hodge test and induction test, were evaluated using cefoxitin-resistant Escherichia coli and Klebsiella pneumoniae isolates. The cefoxitin-Hodge test detected all blaCMY-10, and 97.4% of blaCMY-2 allele-positive isolates, but only 57.3% of blaDHA-1 allele-positive isolates. Induction test with an aztreonam and an amoxicillin-clavulanic acid disk was more sensitive than with cefoxitin disk, which detected 86.6% of blaDHA-1 allele-positive isolates. These phenotypic tests should be useful to screen P-M AmpC β-lactamase-producing E. coli and K. pneumoniae isolates.