Prevalence and characterization of plasmid-mediated quinolone resistance genes in Aeromonas spp. isolated from South African freshwater fish

• Presence of chromosomal- and plasmid-mediated quinolone-resistance mechanisms in Aeromonas.
• Predominant GyrA substitution was Ser-83 → Val, whilst Ser-80 → Ile substitutions were observed in ParC.
• First report of qnrB from fish-associated Aeromonas veronii isolates
• Higher prevalence of qnrB than qnrS was detected in fish-associated Aeromonas.
• qnr presence in Aeromonas may serve as reservoir for dissemination to other aquatic bacteria.

An increasing incidence of multidrug-resistant Aeromonas spp., which are both fish and emerging opportunistic human pathogens, has been observed worldwide. Quinolone–resistant Aeromonas spp. isolates are increasingly being observed in clinical and environmental settings, and this has been attributed primarily to target gene alterations, efflux, and transferable quinolone resistance. Thirty-four Aeromonas spp., obtained from freshwater aquaculture systems, were screened for the presence of GyrA and ParC substitutions, efflux activity and the prevalence of plasmid-mediated quinolone resistance genes, qnr and aac-6′-Ib-cr. Although 44% of isolates were resistant to nalidixic acid, the majority were susceptible to ciprofloxacin and ofloxacin. The predominant GyrA substitution was Ser-83 → Val among Aeromonas veronii isolates whilst Aeromonas hydrophila isolates displayed a Ser-83 → Ile substitution, and Ser-80 → Ile substitutions were observed in ParC. Minimum inhibitory concentrations of fluoro(quinolones) were determined in the presence and absence of the efflux pump inhibitor, phenylalanine-arginine β-naphthylamide (PAβN). Addition of PAβN had no effect on the levels of fluoro(quinolone) resistance observed for these isolates. Although no aac-6′-Ib-cr variant genes were identified, qnrB and qnrS were detected for 41% and 24% of isolates, respectively, by Southern hybridization and confirmed by PCR and sequencing. Quinolone resistance in these fish-associated Aeromonas isolates was related to mutations in the quinolone resistance determining regions of GyrA and ParC and presence of qnrB and qnrS. The presence of qnr alleles in Aeromonas spp. isolates may facilitate high-level fluoroquinolone resistance and potentially serve as reservoirs for the dissemination of qnr genes to other aquatic microbes.