Objective: This study was aimed to determine the presence of plasmid mediated AmpC type beta-lactamases in clinical isolates of Enterobacteriaceae by phenotypic and genotypic methods.

Materials and Methods: Five hundred and fifty three Enterobacteria-cea strains isolated from clinical specimens were included in the study. Strains which had a zone diameter less than 18mm in cefoxitin disc diffusion test were investigated for the presence of AmpC genes by multiplex PCR using MOX, CIT, DHA, ACC, EBC, FOX primers. The isolates were also tested phenotypically for the presence of AmpC beta-lactamases by using 3-aminophenylboronic acid based disc diffusion methods. Strains which were positive for AmpC gene were investigated by double disc synergy test (DDST), inhibitor potentiation disc diffusion (IPDD) and Coudron's disc potentiation test.

Results: In 16 of 46 strains which had cefoxitin inhibition diameter smaller than 18 mm ampC gene was determined by multiplex PCR. EBC type AmpC beta-lactamases were detected in 10 strains (two Klebsiella pneumoniae, eight Escherichiae coli), CIT type in four E.coli strains, MOX type in one E.coli strain and FOX type in other one E.coli strain. While all of the 16 gene positive strains were determined by Coudron's disc potentiation test method, 15 strains were found by IPDD and 13 strains were detected by DDST.

Conclusion: Plasmid mediated AmpC beta-lactamases which spread rapidly, are usually not detected by routine susceptibility tests which might lead to treatment failure. Thus surveillance of these enzymes are of concern especially in the nosocomial setting. PCR based detection tests are the most reliable methods for the determination of these genes, however, phenotypical screening and confirmation tests suitable for use in the routine laboratory are needed. In this study we examined three phenotypic test methods for the determination of plasmid mediated AmpC beta-lactamases and Coudron's disc potentiation test was found to exhibit highest correlation with PCR. Thus Coudron's disc potentiation test may be used as the most suitable, easy, economical and reliable method for the determination of plasmid mediated AmpC beta-lactamases in routine laboratory.