Non-ST131 Escherichia coli from cattle harbouring human-like bla(CTX-M-15)-carrying plasmids

To characterize bla(CTX-M-15)-carrying plasmids and lineages of nine strains of Escherichia coli from cattle.

Plasmid DNA was analysed using PCR-based replicon typing and plasmid sub-typing schemes, restriction fragment length polymorphism, S1 nuclease-PFGE and Southern hybridization. Strains were characterized by PFGE, multilocus sequence typing, phylogenetic grouping and B2-O25b:H4-ST131 (where ST stands for sequence type) clone screening. Susceptibilities to antimicrobials were determined by agar diffusion and resistance genes were characterized by PCR and sequencing.

The bla(CTX-M-15) gene was found on F31:A4:B1/IncFII and F2:A-:B-/IncFII plasmids, which have been reported abundantly in humans. On F31:A4:B1/IncFII plasmids, the bla(CTX-M-15) gene was associated with the bla(TEM-1), bla(OXA-1) and aac(6')-Ib-cr resistance genes. The bla(CTX-M-15) gene was also found on IncI1 plasmids of the CC31 clonal complex, recently identified in the human epidemic and virulent E. coli clone O104:H4. None of the cattle isolates belonged to the human and widespread clone B2-O25b:H4/ST131, but were mostly of new STs and of the phylogenetic groups A (n=4), B1 (n=3) or D (n=2). The E. coli isolates harbouring the bla(CTX-M-15)-carrying plasmids were genetically diverse, and were recovered from different geographical locations and farms and at different times.

This study demonstrates that bla(CTX-M-15)-carrying plasmids from cattle-derived non-ST131 E. coli isolates were highly similar to those found in ST131 E. coli isolates commonly reported in humans. It also exemplifies the key role of plasmids versus clonal dissemination in the spread of the bla(CTX-M-15) gene among cattle, and possibly between E. coli isolates detected in humans and cattle.