Carbapenem non-susceptibility associated with global transcriptional regulator MarA in Escherichia coli

Abstract

Background: The AcrAB-TolC system of Escherichia coli is a multidrug efflux system pumping out a wide variety of substrates. Its expression is regulated by an intricate balance between local (AcrR and AcrS) and global regulators (MarA, SoxS and Rob). Global transcriptional regulators are known to be involved in stress response in bacteria. MarA along activating its own transcription; it activates the transcription of other genes including acrAB, tolC and micF. As carbapenem resistance is an increasing trend in hospital acquired infections. The present study was carried out to observe transcriptional response of marA, soxS and rob against concentration dependent carbapenem stress. Methods and materials: In the present study hospital isolates of Escherichia coli were tested for transcriptional expression of AcrAB-TolC and OmpF. Transcriptional expression of marA, soxS and rob with and without concentration gradient carbapenem stress were determined. Sequencing and cloning of marA, soxS and rob was also performed. Results: Twelve isolates with over-expressed AcrAB-TolC and downregulated OmpF were selected for further study. Among them, overexpression of marA against meropenem was observed in seven isolates. Antimicrobial susceptibility pattern of the clones showed that the zone of inhibition formed by the transformants (mar) was decreased as compared with that of parent strain (DH5α) as well as with the plasmid without gene of interest. Further, MIC range of the clones revealed two-fold or more increase in the inhibitory concentration against ertapenem and imipenem was noted for clone of mar while comparing with the parent strain (DH5α) Further, when the DNA sequences of marAwere compared with the reference strain of E. coli ATCC 25922, it displayed nucleotide alterations at many locations. Four-point mutation and 2 deletion mutations in marA sequence was observed. Conclusion: This study demonstrates the role of the global regulators marA in triggering the overexpression of AcrAB efflux pump system conferring resistance towards carbapenems. MarA plays a major role in efflux activity under carbapenem stress. Regulation of Rob under meropenem stress shows an antibiotic specific response and SoxS deviation in transcriptional expression amid particular concentration, needs further investigation.