Microbiological characterization of plasmid-mediated AmpC ß-lactamases and E. coli hyperproducers: how and why ?

The aim of this study is the evaluation of phenotypic method for the detection of plasmid-mediated AmpC producing Enterobacteriaceae by agar diffusion. We developed a phenotypic method with double disk test (CLSI) and evaluation of synergism between Cloxacillin and/or Boronic Acid with cefotaxime and ceftazidime and cefepime with amoxicillin/clavulanic acid. As reference method for AmpC detection we used a multiplex PCR according to Perez-Perez. Among 7476 Enterobacteriaceae we detected 45 strains: 37 (82.2%) plasmid-mediated AmpC producers, 6 (13.3%) E. coli hyperproducers and 2 E. coli (4.5%) positive for both.The AmpC phenotypic test was positive for all the isolates, showing a typical ghost zone between cloxacil- lin and cephalosporins or boronic acid and cephalosporins.The AmpC multiplex PCR confirmed that 28 P. mirabilis and 7 E. coli harboured a gene belonging to the bla-CMY-LAT family. Sequencing defined the presence of CMY-16 in all P. mirabilis, CMY-2 in E. coli, DHA-1 in 3 K. pneumoniae and FOX in 1 K. pneumoniae and allowed us to identify eight strains as E. coli hyperproducer: six E. coli yielded no amplicon and 2 were also producer of CMY-2. In this study the phenotypic method showed a sensitivity and a specificity of 100%.Waiting for the indica- tion of international authorities, we think this phenotypic screening method could be useful in the routine of microbiological laboratories.