Elimination of the risks of colistin resistance gene (mcr-1) in livestock manure during composting

Since its discovery in Escherichia coli, the emergence and rapid spread of the plasmid-mediated colistin resistance gene mcr-1 have become a public health concern. Livestock manure is a potentially important reservoir of mcr-1 because colistin has been widely used in livestock production. Efforts made to accurately quantify the prevalence of mcr-1 in livestock manure and the dynamic changes therein during thermophilic composting have been few and far between. In this study, mcr-1 in 51 collected samples from four kinds of livestock manures was detected and quantified. In total, 16 manure samples were found to be mcr-1 positive, with a detection frequency of 31% in 51 samples. The numbers of mcr-1 gene copies in 12 positive manure samples with a high prevalence of mcr-1 were 10(7)-10(9) copies/g dry weight. During composting, > 90% of mcr-1 in the manure was eliminated in 15 days at high temperature (44-65 degrees C), and mcr-1 was completely undetectable after 22 days. The reduction of mcr-1 following manure composting may be ascribed to the decreased number of potential mcr-1-harboring bacteria, Enterobacteriaceae and Pseudomonas. The results indicated that thermophilic composting effectively eliminated mcr-1 and inhibited its spread from livestock manure to the environment.