Multidrug resistance in bacteria is generally attributed to the acquisition of multiple transposons and plasmids bearing genetic determinants for different mechanisms of resistance (48, 62). However, descriptions of intrinsic mechanisms that confer multidrug resistance have begun to emerge. The first of these was a chromosomally encoded multiple antibiotic resistance (mar) locus (Fig. 1) in Esc...

Fluoroquinolone-resistant mutants of Mycoplasma hominis were selected in vitro from the PG21 susceptible reference strain either by multistep selection on increasing concentrations of various fluoroquinolones or by one-step selection on agar medium with ofloxacin. The quinolone resistance-determining regions (QRDR) of the structural genes encoding the A and b subunits of DNA gyrase were amplifi...

BACKGROUND AND OBJECTIVES Gonococcal fluoroquinolone resistance is now a significant problem in Japan. We generated gonococcal mutants resistant to norfloxacin in vitro from norfloxacin sensitive isolates and analysed the contribution of three known mechanisms of quinolone resistance in Neisseria gonorrhoeae. MATERIALS AND METHODS Three clinical isolates of N gonorrhoeae susceptible to norflo...

Sir, A recent review by Smith et al.1 and associated correspondence2,3 raised important unresolved issues regarding which quinolones exert dual activity through gyrase and topoisomerase IV in Streptococcus pneumoniae. Dual action involves the substantial engagement of both enzyme killing pathways and is of particular interest in potentially limiting the emergence of resistance. Smith et al.3 co...

OBJECTIVES Quinolone resistance in the opportunistic pathogen Pseudomonas aeruginosa is commonly caused by mutations that alter the target molecules DNA gyrase/topoisomerase IV, or cause activation of various efflux systems. We have analysed the effect of quinolone resistance caused by DNA gyrase/topoisomerase IV mutations on bacterial fitness. METHODS Norfloxacin-resistant mutants were isola...

The genetic basis for fluoroquinolone resistance was examined in 30 high-level fluoroquinolone-resistant Escherichia coli clinical isolates from Beijing, China. Each strain also demonstrated resistance to a variety of other antibiotics. PCR sequence analysis of the quinolone resistance-determining region of the topoisomerase genes (gyrA/B, parC) revealed three to five mutations known to be asso...

The mechanisms of fluoroquinolone resistance in two isolates of Enterobacter cloacae, Ecl#1 and Ecl#2, from the same patient and with identical pulsed-field gel electrophoresis patterns, have been analysed. MICs of ciprofloxacin were 0.25 and 1 mg/L for Ecl#1 and Ecl#2, respectively. Ecl#2 was also more resistant to chloramphenicol and organic solvents. The quinolone resistance determining regi...

Mutants of attenuated Bacillus anthracis with high-level ciprofloxacin resistance were isolated using a three-step in vitro selection. Ciprofloxacin MICs were 0.5 micro g/ml for first-step mutants, which had one of two gyrA quinolone resistance-determining region (QRDR) mutations. Ciprofloxacin MICs were 8 and 16 microg/ml for second-step mutants, which had one of three parC QRDR mutations. Cip...

Alternate mutations in the grlA and gyrA genes were observed through the first- to fourth-step mutants which were obtained from four Staphylococcus aureus strains by sequential selection with several fluoroquinolones. The increases in the MICs of gatifloxacin accompanying those mutational steps suggest that primary targets of gatifloxacin in the wild type and the first-, second-, and third-step...

Development of fluoroquinolone resistance can involve several mechanisms that include chromosomal mutations in genes ( gyrAB and parCE ) encoding the target bacterial topoisomerase enzymes, increased expression AcrAB-TolC efflux system, acquisition transmissible quinolone-resistance genes. In this study, 176 Salmonella isolates from animals with a broad range ciprofloxacin MICs were collected t...