Escherichia coli produces the iron-chelating compound enterobactin to enable growth under iron-limiting conditions. After biosynthesis, enterobactin is released from the cell. However, the enterobactin export system is not fully understood. Previous studies have suggested that the outer membrane channel TolC is involved in enterobactin export. There are several multidrug efflux transporters belonging to resistance-nodulation-cell division (RND) family that require interaction with TolC to function. Therefore, several RND transporters may be responsible for enterobactin export. In this study, we investigated whether RND transporters are involved in enterobactin export using deletion mutants of multidrug transporters in E. coli. Single deletions of acrB, acrD, mdtABC, acrEF, or mdtEF did not affect the ability of E. coli to excrete enterobactin, whereas deletion of tolC did affect enterobactin export. We found that multiple deletion of acrB, acrD, and mdtABC resulted in a significant decrease in enterobactin export and that plasmids carrying the acrAB, acrD, or mdtABC genes restored the decrease in enterobactin export exhibited by the ΔacrB acrD mdtABC mutant. These results indicate that AcrB, AcrD, and MdtABC are required for the secretion of enterobactin.

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.

TolC is required for the function of seven drug efflux systems (AcrAB, AcrD, AcrEF, MdsAB, MdtABC, EmrAB and MacAB) in S. enterica serovar Typhimurium.

AcrAB and AcrEF confer resistance to tigecycline and tetracycline derivatives in Salmonella. RamA and RamR are also involved in resistance to tigecycline in an AcrAB-dependent manner.

A novel metallo-β-lactamase gene, blaIMP-27, was identified in unrelated Proteus mirabilis isolates from two geographically distinct locations in the United States. Both isolates harbor blaIMP-27 as part of the first gene cassette in a class 2 integron. Antimicrobial susceptibility testing indicated susceptibility to aztreonam, piperacillin-tazobactam, and ceftazidime but resistance to ertapenem. However, hydrolysis assays indicated that ceftazidime was a substrate for IMP-27.

Clinical isolates of Klebsiella pneumoniae and Klebsiella oxytoca collected from 20 Japanese medical facilities between 2000 and 2010 were analysed to evaluate the mechanisms of resistance and antibacterial susceptibilities to 14 antimicrobials. Overall, eight of 484 (1.6 %) K. pneumoniae and 19 of 359 (5.3 %) K. oxytoca were determined to be extendedspectrum b-lactamase (ESBL) phenotype isolates, and the identified ESBLs amongst the K. pneumoniae isolates were CTX-M-2, -3, -14 and -15, and SHV-12. In contrast, overproduction of chromosomal b-lactamase OXY-2, which was due to a distinct mutation at the 210 promoter region of this gene, conferred the ESBL phenotype to all the K. oxytoca isolates except one. Based on the Clinical and Laboratory Standards Institute breakpoints, all the ESBL phenotype K. pneumoniae were susceptible to doripenem, flomoxef, moxalactam (latamoxef), cefmetazole and tazobactam/piperacillin, whereas the ESBL phenotype K. oxytoca were susceptible to ceftazidime and ceftibuten in addition to the above, with the exception of tazobactam/piperacillin. Amongst the oral antimicrobials, ceftibuten was relatively effective against both ESBL phenotype Klebsiella species compared with levofloxacin and amoxicillin/ clavulanic acid.

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