Retrospective survey of mcr-1 and mcr-2 in German pig-fattening farms, 2011-2012

- The mcr-1 gene was present in 25.9% of investigated pig farms.
- mcr-1-positive pig husbandries were equally distributed throughout Germany.
- Great heterogeneity among tested mcr-1-positive isolates was detected.
- Colistin resistance genes were predominantly located on IncX4 plasmids.

In November 2015, the first plasmid-encoded colistin resistance gene, mcr-1, was described in animals and in humans in China. Subsequently, a multitude of further studies was performed and quite recently the global spread of mcr-1 as well as the occurrence of a new gene variant, mcr-2, was reported. To obtain an overview of the occurrence of the colistin resistance genes mcr-1 and mcr-2 in German pig farms, a retrospective study, including 436 boot swab and pooled faecal samples collected from 58 pig-fattening farms throughout Germany, was performed. Whilst mcr-2 was not detected, the presence of mcr-1 was confirmed in 43 Escherichia coli isolates from 15 farms, indicating that the mcr-1 gene was present in 9.9% of the analysed samples and 25.9% of the investigated pig farms. Subsequent characterisation of the isolates showed colistin minimum inhibitory concentrations (MICs) of 4-8 µg/mL, with most isolates being resistant to several antibiotics including cephalosporins and/or fluoroquinolones. Pulsed-field gel electrophoresis (PFGE) showed great heterogeneity among the tested mcr-1-positive isolates. However, further analyses of 15 selected E. coli isolates (one per mcr-1-positive farm) indicated that the colistin resistance genes were predominantly located on IncX4 plasmids, highly similar to a plasmid initially isolated from an E. coli derived from a human patient in Brazil. The results described herein support the already expressed concern for public health and further underline the need for monitoring programmes in veterinary practice as well as in human medicine.