Detection of plasmid-mediated ampc in Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis

Abstract

Aims: This study investigated the prevalence of plasmid-mediated ampC in selected clinical isolates of Escherichia coli, Klebsiella species and Proteus mirabilis, and compared the results of boronic-acid disc screening with conventional susceptibility testing for the detection of ampC-positive isolates. Methods: E. coli, Klebsiella species and P. mirabilis with reduced susceptibility to amoxycillin-clavulanate, cefuroxime and cephalexin, but without phenotypic evidence of extended-spectrum beta-lactamases were screened for AmpC activity using enzyme-extraction methods. The presence of plasmid-mediated ampC was determined by multiplex PCR. Antibiotic susceptibilities were determined using both disc and dilution-based methods. A disc-based screening method for detection of AmpC-producing strains was evaluated using boronic acid as an inhibitor of AmpC, and cefoxitin as the antibiotic substrate. Results: Plasmid-mediated ampC was present in 26% of study isolates, with CMY-like enzymes detected predominantly in E. coli and DHA-like enzymes predominantly in K. pneumoniae. Current susceptibility methods failed to detect a significant proportion of plasmid-mediated AmpC-producing isolates, with 33% of such strains interpreted as susceptible to third-generation cephalosporins using current CLSI breakpoints. The boronic-acid disc method showed sensitivity and specificity of 90% and 98% respectively in detecting AmpC-positive isolates. Conclusion: The prevalence of plasmid-mediated ampC was high in the study population, and may be missed by conventional susceptibility testing methods. Inhibitor-based screening methods would improve detection of this emerging resistance phenotype.