Most Escherichia coli strains overproducing chromosomal AmpC b-lactamase belong to phylogenetic group A

Objectives: To determine the phylogenetic group and the production of different virulence factors (VFs) of a collection of Escherichia coli strains overproducing their chromosomal AmpC cephalosporinase. Methods: Fifty-five E. coli strains, isolated over a 12 year period, and previously identified as AmpC overproducers by increased MICs of third-generation cephalosporins without extended-spectrum b-lactamase production (negative double-disc synergy test), were phylogrouped by multiplex PCR. As a comparison, 100 E. coli clinical isolates, susceptible to all b-lactams, were also tested by the same method. The ampC promoter sequence was determined for all these isolates. ERIC-2 PCR (where ERIC stands for enterobacterial repetitive intergenic consensus) was used to compare the isolates. Search for virulence-associated genes (papG alleles, sfa/foc, hly and iucC) was performed by multiplex PCR for the 55 AmpC overproducers. Results: Most of the AmpC overproducers (47/55) belonged to phylogenetic group A, correlated with a low prevalence of the main VFs in these strains. The 232, 242 and 211 mutations, responsible for AmpC overproduction, were usually associated with DNA polymorphisms at positions 288, 282, 218, 11 and 158 in the ampC promoter. In the control susceptible isolates, these polymorphisms were detected in 13 ampC promoters (9 group B1 and 4 group A). These polymorphisms were never associ-ated with the main phylogenetic group B2, representing 66 % of the susceptible isolates. Conclusions: AmpC overproduction was clearly correlated with non-virulent commensal phylogenetic groups A and B1, and absence of the main E. coli VFs. Susceptible isolates harbouring the same sequence polymorphisms as AmpC overproducers also belonged to commensal phylogenetic groups