Prevalence of AmpC β-lactamase in Clinical Isolates of Escherichia coli, Klebsiella spp., and Proteus mirabilis in a Tertiary Hospital in Tehran, Iran

Background: AmpC beta-lactamase confers resistance to a variety of beta-lactam agents, and all plasmid-mediated AmpC genes are considered clinically significant. The transfer of the AmpC gene to plasmid has resulted in dissemination among the Enterobacteriaceae family, including Escherichia coli, Klebsiella spp., and Proteus mirabilis. Objectives: The prevalence of plasmid-mediated AmpC genes was determined in isolates of E. coli, Klebsiella spp., and P. mirabilis with reduced susceptibility to cefoxitin or extended-spectrum cephalosporins by the multiplex PCR method. Methods: A total of 310 consecutive non-duplicate isolates of E. coli, Klebsiella spp., and P. mirabilis were obtained from various clinical specimens. Isolates with positive screening test results were subjected to further molecular evaluation. Results: Fifty isolates were positive on the screening test. Among them, positive PCR reactions were identified in 35/221 and 12/77 isolates of E. coli and Klebsiella spp., respectively, including 16 (34.0%) for CIT only, 7 (14.8%) for DHA only, and 24 (51.0%) for both DHA and CIT. No isolate was positive for FOX or MOX. No Proteus organism was positive for AmpC genes. Conclusions: Currently, phenotypic tests are unable to accurately and reliably recognize plasmid-mediated AmpC beta-lactamase-producing organisms. Although not possible for routine testing, clinical laboratories, especially in referral centers, should employ molecular testing for surveillance studies.