Molecular mechanisms of quinolone resistance in clinical isolates of Aeromonas caviae and Aeromonas veronii bv. sobria

ABSTRACT

Mutations in quinolone targets were studied together with quinolone efflux pump activation and plasmid-mediatedquinolone resistance determinants in nalidixic-acid-resistant isolates of Aeromonas caviae and Aeromonas veronii.Among 135 clinical Aeromonas spp. isolated from stools of patients with gastrointestinal symptoms, 40 nalidixic acid-resistantstrains belonging to A. caviae and A. veronii were selected and their susceptibility to different quinolones (ciprofloxacin,norfloxacin, ofloxacin) further evaluated. Susceptibility to nalidixic acid and ciprofloxacin in the presence/absence of Phe-Arg-β-naphthylamide was also determined. The 16 nalidixic-acid-resistant strains identified as A. caviae were more resistantthan the 24 A. veronii bv. sobria strains to ciprofloxacin, norfloxacin, and ofloxacin. All strains showed a mutation (single ordouble) at position 83 of the QRDR sequence of gyrA, with Ser-83 → Ile as the most frequent substitution. By contrast, nomutations were found at position 87 of gyrA. Double substitutions (GyrA-ParC) were detected in 50% of A. veronii bv. sobriaisolates and in 43.75% of A. caviae strains. Both species showed decreases in the MICs of ciprofloxacin. A qnrS gene wasfound in an A. caviae strain. Thus, in the two species of nalidixic-acid-resistant Aeromonas isolates examined, resistancemediated by efflux pumps contributed only slightly to ciprofloxacin resistance. While two isolates were positive for theaac(6′)-Ib gene, no -cr variants were detected (AU)