Use of a broad-host-range gyrA plasmid for genetic characterization of fluoroquinolone-resistant gram-negative bacteria.

The gyrA genotypes of ciprofloxacin-resistant clinical isolates of Escherichia coli (n = 3), Klebsiella pneumoniae (n = 4), Providencia stuartii (n = 2), Pseudomonas aeruginosa (n = 1), and Acinetobacter calcoaceticus (n = 1) were analyzed in a dominance test. This test is based on the dominance of a wild-type gyrA gene (gyrA+) over the quinolone resistance allele (gyrA) in a heterodiploid strain. Plasmid pBP515, developed to carry the gyrA+ gene of E. coli K-12 on a broad-host-range vector derived from pRSF1010, was used to obtain heterodiploid strains. Plasmid pBP515 encodes kanamycin and gentamicin resistance and is transferable via mobilization by a pRP1-derived helper plasmid (pRP1H) to strains of several gram-negative species. After the introduction of pBP515, single-cell MICs (as measured by reduction of the viable cell count) of ciprofloxacin and nalidixic acid decreased by 4- to greater than 8,000-fold for all strains tested, and 8 of the 11 strains regained ciprofloxacin susceptibilities similar to those of the respective wild types. The results indicate that (i) high-level fluoroquinolone resistance in clinical isolates of E. coli, K. pneumoniae, P. aeruginosa, and A. calcoaceticus can result from mutational alteration of the gyrA gene, and (ii) gyrA mutations are involved in high levels of fluoroquinolone resistance in P. stuartii. Additional mutations outside the gyrA locus may contribute to resistance in K. pneumoniae and P. stuartii.