Subinhibitory concentration stress of colistin enhanced PhoPQ expression in Escherichia coli harboring mcr‐1

The increased and inappropriate use of colistin led to the emergence of its resistance among Gram‐negative bacterial isolates and the most common mechanism of colistin resistance in Gram‐negative bacteria is the modification of the lipopolysaccharide mediated by two‐component regulatory systems, PhoPQ and PmrAB. The aim of the present study was to investigate the transcriptional expression of the PhoPQ system against colistin stress in clinical isolates of Escherichia coli with colistin‐resistant phenotype. Six colistin‐resistant E. coli isolates were obtained from Silchar Medical College and Hospital, Silchar that were of clinical origin and received for routine culture and sensitivity testing. Screening for colistin resistance was done by broth microdilution method and further screened for the presence of the different types of plasmid‐mediated colistin resistance mcr genes namely, mcr‐1 to mcr‐10 by polymerase chain reaction (PCR). The screened positive isolates were subjected to PCR assay targeting phoP and phoQ genes and their expression was measured by quantitative real‐time PCR. The results of this study revealed that two E. coli isolates (TS2 and TS4) were found to carry the mcr‐1 gene. PhoP and PhoQ gene amplification was observed in all the isolates. Transcriptional analysis showed that the isolates harboring the mcr‐1 gene showed an enhanced level of expression in the PhoP, PhoQ genes in the presence of a subinhibitory concentration of colistin whereas no significant expression was observed for the isolates which were devoid of the mcr gene. This study demonstrates the involvement of mcr‐1 in the PhoPQ system in clinical isolates of colistin‐resistant E. coli which will help in designing a molecular marker for detecting colistin‐resistant E. coli and contribute to the assessment of resistance burden and infection control strategy.