Antimicrobial resistance is rapidly expanding, in a large part due to mobile genetic elements. We screened 94 fecal fluoroquinolone-resistant Escherichia coli isolates from Nigeria for six plasmid-mediated quinolone resistance (PMQR) genes. Sixteen isolates harbored at least one of the PMQR genes and four were positive for aac-6-Ib-cr. In one strain, aac-6-Ib-cr was mapped to a 125 Kb self-transmissible IncFII plasmid, pMB2, which also bears blaCTX-M-15, seven other functional resistance genes and multiple resistance pseudogenes. Laboratory strains carrying pMB2 grew faster than isogenic strains lacking the plasmid in both rich and minimal media. We excised a 32 Kb fragment containing transporter genes and several open-reading frames of unknown function. The resulting 93 Kb mini-plasmid conferred slower growth rates and lower fitness than wildtype pMB2. Trans-complementing the deletion with the cloned sitABCD genes confirmed that they accounted for the growth advantage conferred by pMB2 in iron-depleted media. pMB2 is a large plasmid with a flexible resistance region that contains loci that can account for evolutionary success in the absence of antimicrobials. Ancillary functions conferred by resistance plasmids can mediate their retention and transmissibility, worsening the trajectory for antimicrobial resistance and potentially circumventing efforts to contain resistance through restricted use.

We discuss how Kelsh et al. methodically reduced the relevant cases by following overly stringent criteria for inclusion. Am. J. Ind. Med. 60:152-162, 2017. © 2017 Wiley Periodicals, Inc.

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Antimicrobial resistance is rapidly expanding, in a large part due to mobile genetic elements.We screened 94 fecal fluoroquinolone-resistant Escherichia coli isolates from Nigeria for six plasmid-mediated quinolone resistance (PMQR) genes. Sixteen isolates harbored at least one of the PMQR genes and four were positive for aac-6-Ib-cr. In one strain, aac-6-Ib-cr was mapped to a 125 Kb self-transmissible IncFII plasmid, pMB2, which also bears bla CTX-M-15 , seven other functional resistance genes and multiple resistance pseudogenes. We hypothesized that pMB2 had been selected by antimicrobials and that its large size would confer a growth disadvantage.However, laboratory strains carrying pMB2 grew at least as fast as isogenic strains lacking the plasmid in both rich and minimal media. We excised a 32 Kb fragment containing the sitABCD and another putative transporter, pefB, a papB homolog, and several open-reading frames of unknown function. The resulting 93 Kb mini-plasmid conferred slower growth rates and lower fitness than wildtype pMB2. Trans-complementing the deletion with the cloned sitABCD genes confirmed that they accounted for the growth advantage conferred by pMB2 in iron-depleted media. The mini-plasmid additionally conferred autoaggregation and was less transmissible and both phenotypes could be complemented with a pefB clone. pMB2 is a large plasmid with a flexible resistance region that contains multiple loci that can account for evolutionary success in the absence of antimicrobials. Ancillary functions conferred by resistance plasmids can mediate their retention and transmissibility, worsening the trajectory for antimicrobial resistance and potentially circumventing efforts to contain resistance through restricted use.

Expression of the quinolone resistance gene is increased by quinolones, but unlike induction of some other genes, the bacterial SOS system is not involved and no box is found upstream. Nonetheless, at least 205 bp of upstream sequence is required for induction to take place. An upstream sequence bound to beads trapped potential binding proteins from cell extracts that were identified by mass spectrometry as Dps, Fis, Ihf, Lrp, CysB, and YjhU. To further elucidate their role, a reporter plasmid linking the upstream sequence to was introduced into cells of the Keio collection with single-gene deletions and screened for expression. Mutants in and had decreased induction, while induction in a mutant was increased and ,, ,, and other mutants showed no change. The essential upstream sequence contains potential binding sites for Ihf and DnaA. A deletion could not be tested because it provides essential functions in cell replication; however, increased expression decreased induction while decreased expression enhanced it, implying a role for DnaA as a repressor. In a mobility shift assay, purified IhfA, IhfB, and DnaA proteins (but not CysB) were shown to bind to the upstream segment. Induction decreased in a quinolone-resistant mutant, indicating that GyrA also has a role. Thus, quinolones acting through proteins DnaA, GyrA, IhfA, and IhfB regulate expression of.