21.11.2015

Liu et al (2015) The Lancet Infectious Diseases

Colistin belongs to the family of polymyxins, cationic polypeptides, with broad-spectrum activity against Gram negative bacteria, including most species of the family Enterobacteriaceae. The two polymyxins currently in clinical use are polymyxin B and polymyxin E (colistin), which differ only by one amino acid from each other and have comparable biological activity. The mechanism of resistance to polymyxins is modification of lipid A, resulting in reduction of polymyxin affinity. Until now, all reported polymyxin resistance mechanisms are chromosomally mediated, and involve modulation of two component regulatory systems leading to modification of lipid A with moieties such as phosphoethanolamine or 4-amino-4-arabinose, or in rare instances total loss of the lipopolysaccharide. Thus far, the polymyxins remain one of the last classes of antibiotics in which resistance is not known to spread from cell to cell (ie, plasmid mediated). During a routine surveillance project on antimicrobial resistance in commensal Escherichia coli from food animals in China, a major increase of colistin resistance was observed. When an E coli strain, SHP45, possessing colistin resistance that could be transferred to another strain, was isolated from a pig. The new research reports the first case of a plasmid-mediated colistin resistance mechanism, designated MCR-1. The researchers describe its putative structure, mechanism of action, and its emergence in Enterobacteriaceae from animal and human isolates, and provide evidence for the spread of mcr-1 from the veterinary sector to human beings.  the researchers report finding the gene in 166 of 804 pigs at slaughter across four provinces, and from pork and chicken sold in 30 open markets and 27 supermarkets in Guangzhou between 2011 and 2014. It was also found in 1 per cent of 1,322 samples they tested from hospitalized patients in China, which the researchers called a relatively low proportion. The resistance had spread between a range of bacterial strains and species, including E. coli, Klebsiella pneumoniae and Pseudomonas aeruginosa.