mcr-1 and mcr-2 variant genes identified in Moraxella species isolated from pigs in Great Britain from 2014 to 2015

Abstract

Objectives: To determine the occurrence of mcr-1 and mcr-2 genes in Gram-negative bacteria isolated from healthy pigs in Great Britain.

Methods: Gram-negative bacteria (n = 657) isolated from pigs between 2014 and 2015 were examined by WGS.

Results: Variants of mcr-1 and mcr-2 were identified in Moraxella spp. isolated from pooled caecal contents of healthy pigs at slaughter collected from six farms in Great Britain. Other bacteria, including Escherichia coli from the same farms, were not detected harbouring mcr-1 or mcr-2. A Moraxella porci-like isolate, MSG13-C03, harboured MCR-1.10 with 98.7% identity to MCR-1, and a Moraxella pluranimalium-like isolate, MSG47-C17, harboured an MCR-2.2 variant with 87.9% identity to MCR-2, from E. coli; the isolates had colistin MICs of 1-2 mg/L. No intact insertion elements were identified in either MSG13-C03 or MSG47-C17, although MSG13-C03 harboured the conserved nucleotides abutting the ISApl1 composite transposon found in E. coli plasmids and the intervening ∼2.6 kb fragment showed 97% identity. Six Moraxella osloensis isolates were positive for phosphoethanolamine transferase (EptA). They shared 62%-64.5% identity to MCR-1 and MCR-2, with colistin MICs from 2 to 4 mg/L. Phylogenetic analysis indicated that MCR and EptA have evolved from a common ancestor. In addition to mcr, the β-lactamase gene, blaBRO-1, was found in both isolates, whilst the tetracycline resistance gene, tetL, was found in MSG47-C17.

Conclusions: Our results add further evidence for the mobilization of the mcr-pap2 unit from Moraxella via composite transposons leading to its global dissemination. The presence of mcr-pap2 from recent Moraxella isolates indicates they may comprise a reservoir for mcr.

Figure 1.

Phylogenetic tree of EptA, MCR-1-like and MCR-2-like amino acid sequences identified in Moraxella species and those present in other bacteria.

Figure 2.

(a) Chromosomal arrangement of eptA, mcr-1-like and mcr-2-like genes in Moraxella species. mcr and other flanking genes have been annotated; hypothetical genes have been left blank. The shadow parallelograms between each sequence denote sequence identity. (b) Alignment of the mcr-1.10-pap2 unit in M. porci-like MSG13-C03 with E. coli plasmids harbouring mcr-1-pap2. Alignment of the mcr-1-pap2 region in E. coli plasmid pECJS-59-244 (KX084394), M. porci-like MSG13-C03 and E. coli KX528699. The upstream and downstream flanking sequences are shown, with the underlined bases indicating 100% identity, and the conserved trinucleotide and dinucleotide sequences are shown in red.