Most Escherichia coli strains overproducing chromosomal AmpC β-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 β-lactamase production (negative double-disc synergy test), were phylogrouped by multiplex PCR. As a comparison, 100 E. coli clinical isolates, susceptible to all β-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 − 32, −42 and − 11 mutations, responsible for AmpC overproduction, were usually associated with DNA polymorphisms at positions − 88, − 82, −18, +1 and + 58 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 associated 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.