Molecular characterization of AmpC-producing Escherichia coli clinical isolates recovered in a French hospital.

OBJECTIVES: To characterize the AmpC-type beta-lactamases produced by Escherichia coli clinical isolates.
METHODS: E. coli isolates recovered in a French hospital in 2006 were selected on the basis of a resistance phenotype consistent with increased AmpC production. The presence of genes coding for plasmid-mediated cephalosporinases as well as the existence of mutations in the chromosome-borne ampC genes was studied by PCR and sequencing. Genes for chromosomal cephalosporinases were cloned and the conferred resistance patterns were analysed. The isolates were submitted to phylotyping and genotyping analysis.
RESULTS: Thirty-four out of 2800 E. coli isolates were selected. Sixteen isolates, which overexpressed their chromosomal wild-type cephalosporinases due to mutations into their promoter sequence, were susceptible to extended-spectrum cephalosporins (ECLs). Eighteen isolates, mostly of the commensal phylogenetic group A or B1, had reduced susceptibility to ECLs, due to the production of chromosomal extended-spectrum AmpC (ESAC) beta-lactamases, or plasmid-mediated cephalosporinases (CMY-2 and ACC-1), or to combined mechanisms of resistance. Sequence analysis showed that ESAC beta-lactamases had amino acid changes in the R2 binding site, among which was a novel structural change corresponding to the duplication of Ile-283 in the H-9 helix. All the E. coli clinical isolates were non-clonally related except for four CMY-2-producing strains.
CONCLUSIONS: This work sheds new light on the spread of ESAC beta-lactamases in E. coli. It showed that this emerging mechanism of resistance could be as frequent as plasmid-mediated cephalosporinases (0.21% and 0.28% of the E. coli isolates, respectively) and that a phenotypic approach is not able to identify these mechanisms of resistance.