Emergence of mcr-1 conferred colistin resistance among bacterial isolates from urban sewage water in India.

Increased use of colistin, a last resort drug due to failure of carbapenems, has possibly contributed in development and spread of resistance to colistin among Enterobacteriaceae. The colistin belongs to the family of polymyxins, cationic polypeptides, with broad-spectrum activity against Gram-negative bacteria. In this study, we obtained 253 non-duplicate bacterial isolates from sewage water in Delhi and phenotypically screened for colistin resistance. Of the 47 positive isolates, the colistin resistance gene mcr-1 was detected among 5 isolates. Based on 16S ribosomal RNA-based identification, bacterial isolates were found to be Escherichia coli, Aeromonas veronii, and Aeromonas dhakensis. Extended spectrum β-lactamases (ESBL)-resistant determinants CTX-M and TEM were detected in all five mcr-1 positive isolates. On the basis of literature survey, this is the first report of mcr-1 gene from Aeromonas veronii and Aeromonas dhakensis worldwide. Furthermore, mcr-1 gene has not been reported earlier from sewage water in India. Antibiotic susceptibility test of all five isolates against 9 different classes of drugs revealed multidrug-resistant phenotype with high minimum inhibitory concentration values. In vitro transconjugation studies showed successful transfer of mcr-1 and other ESBL-resistant determinants. The occurrence of colistin resistance phenotype conferred by plasmid-based mcr-1 gene in the environment and an ever-increasing list of bacterial isolates is a cause of concern. A comprehensive survey of different water bodies and epidemiological studies are required to assess the risk of dissemination of resistance determinants.