Diversity of CTX-M-1-producing E.coli from German food samples and genetic diversity of the blaCTX-M-1 region on IncI1 ST3 plasmids

Antimicrobial resistance to cephalosporins is commonly mediated by extended-spectrum beta-lactamases (ESBL) or plasmidic AmpC beta-lactamases (pAmpC). In livestock bla(CTX-M-1) is the most frequently detected ESBL-encoding gene. As transmission to consumers through contaminated food is often proposed, this study characterized ESBL/pAmpC-producing E. cob collected from food samples. Therefore, samples from food products of animal origin and vegetables were screened for phenotypically resistant E. coli by selective cultivation. The ESBL genotype was confirmed for 404 isolates with the majority of them (n = 212) harboring the bla(CTX-M-1) gene. PFGE and MLST analyses as well as plasmid characterization were carried out for 89 isolates, selected under epidemiological aspects. In addition, 44 isolates were investigated by whole genome sequencing and/or sequencing of their plasmids on an Illumina Miseq platform. MLST and PFGE indicated a diverse population of CTX-M-1-producing E. coli in German food samples with no spread of single clonal lineages. The majority of the isolates harbored the bla(CTX-M-1) gene on IncI1 plasmids. Frequently, the gene was associated with the ISEcp1 element and located on a similar to 100 kb IncI1 plasmid depicting the plasmid multilocus sequence type (ST) 3. The bla(CTX-M-1) gene and its flanking sequences were located within the shufflon of the type IV pilus region in diverse orientations. In conclusion, dissemination of the CTX-M-1 beta-lactamase within food samples of animal origin is driven by the transmission of a similar to 100 kb large IncI1 ST3 plasmid. Apart from conjugal transfer of IncI1 ST3 plasmids the transmission of the bla(CTX-M-1) gene might be further promoted through mobilization due to its location within a recombination hot-spot of IncI1 plasmids.