The Up-regulated Multidrug Efflux Pump MdtEF Protects Against Nitrosative Damages during the Anaerobic Respiration of E. coli

Abstract

Background: Drug efflux represents an important protection mechanism against antibiotics and toxic compounds in bacteria. Efflux genes constitute from 6% to 18% of all transporters in bacterial genomes, yet their regulation, natural substrates, and physiological functions are poorly understood. Among the 20 chromosomally encoded efflux genes in E. coli K-12, only the AcrAB-TolC system is constitutively expressed during the ordinary laboratory growth of E. coli. Methods: To explore conditions and circumstances that trigger the expression of additional efflux genes as well as their physiological functions, we examined expression of all 20 efflux genes in a physiologically relevant circumstance for E. coli: anaerobic conditions. Results: We found that transcription, mRNA, and protein levels of mdtEF gene products are up-regulated more than 20 fold under anaerobic conditions than aerobic conditions. Consistent with the up-regulation of this efflux system, ΔmdtEF renders a declined drug tolerance in an drug sensitive KAM3 strain under anaerobic conditions, whereas no defect was observed in ΔmdtEF strain when cells are cultured aerobically, suggesting the up-regulated MdtEF efflux pump is active in anaerobic growth of E. coli. Since the up-regulation of mdtEF by anaerobic growth occurs in the absence of antibiotics and drugs, we speculate that MdtEF has physiological functions under this growth conditions. To explore this, we compared viability of ΔmdtEF and WT MG1655 strains and found that ΔmdtEF caused a decreased cell survival during prolonged anaerobic growth of E. coli. This defect became more pronounced when cells grow in the presence of 10mM nitrate, but no defect was observed in ΔmdtEF strain when cells grow in the presence of 40 mM fumarate under the same anaerobic conditions, suggesting that MdtEF has physiological roles relevant to the anaerobic respiration of nitrate. We show that MdtEF export nitrosated cellular components such as 3-nitrosoindole derivatives caused by reactive nitrogen species and indole accumulation during anaerobic growth of E. coli, thus detoxifying the damages caused by these substances. Conclusions: We propose that E. coli up-regulates the efflux pump MdtEF to protect it from nitrosative damages during anaerobic respiration of nitrate.