Molecular epidemiology of isolates with multiple mcr plasmids from a pig farm in Great Britain: the effects of colistin withdrawal in the short and long term.

Duggett NA1, Randall LP1, Horton RA1, Lemma F1, Kirchner M1, Nunez-Garcia J1, Brena C2, Williamson SM3, Teale C4, Anjum MF1.

Author information

1: Animal and Plant Health Agency, Weybridge, New Haw, Addlestone, Surrey, UK.
2: Animal and Plant Health Agency (Thirsk), West House, Station Road, Thirsk, North Yorkshire, UK.
3: Animal and Plant Health Agency (Bury St Edmunds), Rougham Hill, Bury St Edmunds, Suffolk, UK.
4: Animal and Plant Health Agency (Shrewsbury), Kendal Road, Shrewsbury, UK.

Abstract

BACKGROUND:

The environment, including farms, might act as a reservoir for mobile colistin resistance (mcr) genes, which has led to calls for reduction of usage in livestock of colistin, an antibiotic of last resort for humans.

OBJECTIVES:

To establish the molecular epidemiology of mcr Enterobacteriaceae from faeces of two cohorts of pigs, where one group had initially been treated with colistin and the other not, over a 5 month period following stoppage of colistin usage on a farm in Great Britain; faecal samples were also taken at ∼20 months.

METHODS:

mcr-1 Enterobacteriaceae were isolated from positive faeces and was WGS performed; conjugation was performed on selected Escherichia coli and colistin MICs were determined.

RESULTS:

E. coli of diverse ST harbouring mcr-1 and multiple resistance genes were isolated over 5 months from both cohorts. Two STs, from treated cohorts, contained both mcr-1 and mcr-3 plasmids, with some isolates also harbouring multiple copies of mcr-1 on different plasmids. The mcr-1 plasmids grouped into four Inc types (X4, pO111, I2 and HI2), with mcr-3 found in IncP. Multiple copies of mcr plasmids did not have a noticeable effect on colistin MIC, but they could be transferred simultaneously to a Salmonella host in vitro. Neither mcr-1 nor mcr-3 was detected in samples collected ∼20 months after colistin cessation.

CONCLUSIONS:

We report for the first known time on the presence in Great Britain of mcr-3 from MDR Enterobacteriaceae, which might concurrently harbour multiple copies of mcr-1 on different plasmids. However, control measures, including stoppage of colistin, can successfully mitigate long-term on-farm persistence.