Background: The issue of carbapenem resistance in E.coli is very concerning and it is speculated that cumulative effect of both primary resistance genes and secondary resistance genes that act as helper to the primary resistance genes are the reason behind their aggravation. Therefore, here we attempted to find the role of two secondary resistance genes (SRG) ccdB and repA2 in carbapenem resistance in E. coli (CRE). In this context influential genes belonging to secondary resistome that act as helper to the primary resistance genes like blaNDM and blaCTX-M in aggravating β-lactam resistance were selected from an earlier reported in silico study. Transcriptional expression of the selected genes in clinical isolates of E.coli that were discretely harboring blaNDM-1, blaNDM-4, blaNDM-5, blaNDM-7 and blaCTX-M-15 with and without carbapenem and cephalosporin stress (2 μg/ml) was determined by real time PCR. Cured mutants sets that were lacking (i) primary resistance genes, (ii) secondary resistance genes and (iii) both primary and secondary resistance genes were prepared by SDS treatment. These sets were then subjected to antibiotic susceptibility testing by Kirby Bauer disc diffusion method.
Results: Out of the 21 genes reported in the in silico study, 2 genes viz. repA2 and ccdB were selected for transcriptional expression analysis. repA2, coding replication regulatory protein, was downregulated in response to carbapenems and cephalosporins. ccdB, coding for plasmid maintenance protein, was also downregulated in response to carbapenems except imipenem and cephalosporins. Following plasmid elimination assay increase in diameter of zone of inhibition under stress of both antibiotics was observed as compared to uncured control hinting at the reversion of antibiotic susceptibility by the-then resistant bacteria.
Conclusion: SRGs repA2 and ccdB help sustenance of blaNDM and blaCTX-M under carbapenem and cephalosporin stress.
Keywords: Carabapenem; Cephalosporin; Escherichia coli; Secondary resistance; ccdB; repA2.