Roles of gyrA mutations in resistance of clinical isolates and in vitro mutants of Bacteroides fragilis to the new fluoroquinolone trovafloxacin

We determined whether gyrA mutations were present in fluoroquinolone-resistant laboratory mutants derived from the Bacteroides fragilis reference strain ATCC 25285 and in clinical isolates of B. fragilis. The two first-step mutants selected on ciprofloxacin (CIP) were devoid of gyrA mutations, whereas two of the three CIP-selected second-step mutants studied presented the same gyrA mutation leading to a Ser82Phe change. Unusual GyrA alterations, Asp81Asn or Ala118Val, were detected in two of the three first step mutants selected on trovafloxacin (TRO), Mt3 and Mt1, respectively. The Ala118Val change had no effect on the susceptibility of Mt1 to CIP, No second-step mutant could be obtained with TRO as a selector. For the 12 clinical isolates studied, a Ser82Phe change in GyrA was found only in the 3 strains which showed the highest levels of TRO resistance (MIC, 4 mu g/ml). Thus, the resistance phenotypes and genotypes observed in fluoroquinolone-resistant clinical isolates of B. fragilis were similar to those found in CIP-selected laboratory mutants, whereas peculiar mutational events could be selected in vitro with TRO.