Molecular characterization of plasmids encoding bla from faecal Escherichia coli in travellers returning to the UK from South Asia.

Background: The global prevalence of extended-spectrum beta-lactamase-producing Escherichia coli is rising and is dominated by bla spread by plasmids. Travellers to South Asia from Western Europe have high rates of acquisition of faecal CTX-M-producing E. coli (CTX-M-EC).

Aims: To determine the conjugative ability of CTX-M-EC acquired by healthy volunteers after travel to South Asia, the proportion of travel-acquired CTX-M-EC where bla is encoded on a plasmid vs on the bacterial chromosome, and the relatedness of travel-acquired CTX-M-EC plasmids to previously sequenced plasmids.

Methods: Faecal samples were collected pre- and post-travel from 23 volunteers who visited South Asia, and CTX-M-EC were cultured. After short- and long-read sequencing, 10 plasmid sequences were identified and compared with previously sequenced plasmids in GenBank. Conjugation to E. coli K-12 was undertaken using filter mating.

Findings: Thirty-five percent of CTX-M-EC isolates tested transferred the bla plasmid by conjugation. Travel-acquired CTX-M-EC carried bla on a plasmid in 62% of isolates, whereas 38% of isolates had bla on the chromosome. CTX-M-EC plasmids acquired after travel to South Asia had close homology to previously described epidemic plasmids which are widely disseminated in humans, animals and the natural environment.

Conclusion: Globally successful epidemic plasmids are involved in the spread of CTX-M-EC. Targeted strategies may be used to displace such plasmids from the host strain as part of efforts in infection prevention and control in healthcare settings. Bacteria with bla plasmids were readily acquired by healthy volunteers, and were carried on return to the UK, providing opportunities for onward dissemination.