Cross-resistance to fluoroquinolones in multiple-antibiotic-resistant (Mar) Escherichia coli selected by tetracycline or chloramphenicol: decreased drug accumulation associated with membrane changes in addition to OmpF reduction

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Abstract
Chromosomal multiple-antibiotic-resistant (Mar) mutants of Escherichia coli, selected on agar containing low concentrations of tetracycline or chloramphenicol, were 6- to 18-fold less susceptible to the fluoroquinolones than were their wild-type E. coli K-12 or E. coli C parental strains. The frequency of emergence of such mutants was at least, 1,000-fold higher than that of those selected by the fluoroquinolone norfloxacin directly. When Mar mutants, but not wild-type cells, were plated on norfloxacin, mutants resistant to high levels of norfloxacin (2 .mu.g/ml) appeared at a relatively high (.apprx.10-7) frequency. In addition to decreased amounts of OmpF, Mar mutants had other outer membrane protein changes and were four- to eightfold less susceptible to fluoroquinolones than was an ompF::Tn5 mutant lacking only OmpF. Accumulation of [3H]norfloxacin was more than threefold lower in the Mar mutants than in wild-type cells and twofold lower than in the OmpF-deficient derivative. These differences were not attributable to a change in the endogenous active efflux system for norfloxacin in E. coli. Norfloxacin-induced inhibition of DNA synthesis was threefhold lower in intact cells of a Mar mutant than in susceptible cells, but this difference was not seen in toluene-permeabilized cells. Insertion of Tn5 into marA (min 34.05 on the chromosome) led to a return of the wild-type patterns of norfloxacin accumulation, fluoroquinolone and other antimicrobial agent susceptibilities, and outer membrane protein profile, including partial restoration of OmpF. These findings together suggest that marA-dependent fluoroquinolone resistance is linked to decreased cell permeability, only part of which can be accounted for by the reduction in OmpF. Once mutated to marA, cells can achieve high levels of quinolone resistance at a relatively high frequency.