Colistin exposure enhances expression of eptB in colistin-resistant Escherichia coli co-harboring mcr-1

Colistin resistance has increased due to the increasing and inappropriate use of this antibiotic. The mechanism involves modification of lipid A with phosphoethanolamine (PEtN) and/or 4-amino-4deoxy-l-arabinose (L-Ara4N). EptA and eptB catalyze the transfer of phosphoethanolamine to lipid A. In this study, gene network was constructed to find the associated genes related to colistin resistance, and further in vitro validation by transcriptional analysis was performed. In silico studies showed that eptB gene is a highly interconnected node in colistin resistance gene network. To ascertain these findings twelve colistin-resistant clinical isolates of Escherichia coli were selected in which five were harboring the plasmid-mediated mcr-1. Screening for colistin resistance was performed by broth microdilution (BMD) method and Rapid polymyxin NP test. PCR confirmed the presence of the eptA and eptB genes in all isolates and five isolates were harboring mcr-1. Transcriptional expression in five isolates harboring mcr-1, showed an enhanced expression of eptB when exposed under sub-inhibitory colistin stress. The present study for the first time highlighted genetic interplay between mcr-1 and eptA and eptB under colistin exposure.