The Multidrug Efflux Pump MdtEF Protects against Nitrosative Damage during the Anaerobic Respiration in Escherichia coli

Abstract

Drug efflux represents an important protection mechanism in bacteria to withstand antibiotics and environmental toxic substances. Efflux genes constitute 6 -18% of all transporters in bacterial genomes, yet the expression and functions of only a handful of them have been studied. Among the 20 efflux genes encoded in the Escherichia coli K-12 genome, only the AcrABTolC system is constitutively expressed. The expression, activities, and physiological functions of the remaining efflux genes are poorly understood. In this study we identified a dramatic up-regulation of an additional efflux pump, MdtEF, under the anaerobic growth condition of E. coli, which is independent of antibiotic exposure. We found that expression of MdtEF is upregulated more than 20-fold under anaerobic conditions by the global transcription factor ArcA, resulting in increased efflux activity and enhanced drug tolerance in anaerobically grown E. coli. Cells lacking mdtEF display a significantly decreased survival rate under the condition of anaerobic respiration of nitrate. Deletion of the genes responsible for the biosynthesis of indole, tnaAB, or replacing nitrate with fumarate as the terminal electron acceptor during the anaerobic respiration restores the decreased survival of ⌬mdtEF cells. Moreover, ⌬mdtEF cells are susceptible to indole nitrosative derivatives, a class of toxic byproducts formed and accumulated within E. coli when the bacterium respires nitrate under anaerobic conditions. Taken together, we conclude that the multidrug efflux pump MdtEF is up-regulated during the anaerobic physiology of E. coli to protect the bacterium from nitrosative damage through expelling the nitrosyl indole derivatives out of the cells.

Copper Efflux Is Induced during Anaerobic Amino Acid Limitation in Escherichia coli To Protect Iron-Sulfur Cluster Enzymes and Biogenesis

“…This result is in agreement with a recent study with Mycobacterium tuberculosis which demonstrated that an outer membrane copper efflux protein, MctB, but not the inner membrane pump CptV or the multicopper-binding protein MymT is responsible for the full virulence of M. tuberculosis in the hypoxic granulomatous lesions observed in guinea pig (50). These findings, together with the increasing evidence of the roles of efflux pumps in bacterial stress responses (30) and pathogenicity (4), highlight the importance of drug efflux for bacteria. Such physiological roles of efflux pumps in stress responses, combined with their established roles in antibiotic resistance, suggest that…”

“…2C), strongly suggesting that the induction of the Cus system was due to the elevated free Cu(I) from the intracellular copper pool. To exclude the possibility that the change of the induction of the Cus system in the presence of these compounds was unspecific or indirect, we also measured the transcription of an unrelated gene, mdtE, which is activated by the two-component system AcrAB (30). It was shown that the presence or absence of these compounds had no effect on the transcription of P mdtE -lacZ cultured under anaerobic amino acid limitation (see Fig.…”

Analysis of Shigella flexneri Resistance, Biofilm Formation, and Transcriptional Profile in Response to Bile Salts

“…Also, the yegB gene was induced 2.8-fold in the presence of bile salts. This gene is a homolog to the efflux pump component mdtE, which has been shown to protect E. coli from toxic by-product accumulation during anaerobic respiration (60). In all, the induction of genes that support both virulence and exposure to an anaerobic environment further suggests that Shigella utilizes bile as a signal to prepare for infection in the low-oxygen environment of the colon.…”

The Challenge of Efflux-Mediated Antibiotic Resistance in Gram-Negative Bacteria

“…AcrD is an aminoglycoside efflux pump that works with AcrA and TolC (23). MdtF (YhiV) is likely involved in the extrusion of toxic metabolites during nitrosative stress, such as the nitrosyl derivative of indole, produced during anaerobic growth of E. coli (126). MdtBC is unusual because it contains two different transporter proteins, MdtB and MdtC, and appears to function only as a B 2 C heterotrimer (127).…”