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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 regions of gyrA/B and parC/E, and the marORA and acrB genes, were sequenced. Expression of marR, acrB, soxS, robA, ramA and fis was analysed by northern blotting. The activity of a 90 bp E. cloacae mar promoter fragment was examined with the reporter plasmid pIGJ-1mar. Sequencing the gyrAB and parCE genes revealed a single amino acid substitution in GyrA (corresponding to position 83 in GyrA of Escherichia coli) in Ecl#1 and Ecl#2 (Phe83) compared with reference strain E. cloacae DSMZ 3264 (Thr83). Ecl#2 accumulated significantly less norfloxacin and displayed higher levels of expression of marR and acrB than Exl#1, indicative of greater fluoroquinolone efflux activity. Sequencing gyrB, parC/E and marORA, and northern blotting of robA, ramA and fis, did not reveal any further differences between the two strains. No homologue of soxRS was detected in E. cloacae. Expression of GFP from pIGJ1-mar in Ecl#2 was higher than in Ecl#1. In these two closely related clinical isolates of E. cloacae, a target mutation in GyrA (Ecl#1 and Ecl#2) and increased fluoroquinolone efflux by AcrAB (Ecl#2) contribute to the resistance phenotypes, corroborating findings in vitro and in vivo about the sequential development of fluoroquinolone resistance.