Quinolone Resistance and Prevalence of the Related Genes in Photobacterium damselae subsp. damselae Recovered from Diseased Fish in Eastern China

Photobacterium damselae subsp. damselae is a well-recognized marine animal pathogen. Herein, 70 P. damselae subsp. damselae isolates were investigated for quinolone susceptibility and prevalence of the genes including quinolone resistance-determining regions (QRDRs) and plasmid-mediated quinolone resistance (PMQR) genes. A total of 18/70 isolates exhibited high-level resistance, and 23/70 isolates exhibited moderate resistance according to the MIC values. QRDR analysis showed that double mutants in both GyrA (Ser83Ile) and ParC (6/17 Ser80Phe or 11/17 Ser80Tyr) were detected in 94.4% (17/18) high-level quinolone resistance P. damselae subsp. damselae strains. PMQR detection showed that 60.0% (42/70) carried at least one PMQR (1/42 qnrB coexistence with aac(6′)-Ib-cr, 1/42 qnrS coexistence with aac(6′)-Ib-cr, 44/46 qnrS). QnrA, QnrC, qnrD and qepA were not detected in all strains. Among the 42 PMQR-positive strains, 24 showed fluoroquinolones MICs ≤ 0.5 mg/L and 13 MICs ≥ 2 mg/L, all carrying QRDR mutations. For the twenty-eight non-PMQR strains, twenty-three showed fluoroquinolone MICs ≤ 0.5 mg/L without QRDR mutations, and five MICs ≥ 2 mg/L carrying QRDR mutations. In conclusion, qnrS (qnrS2 allele) is the major PMQR widespread in P. damselae subsp. damselae isolated from eastern China; however, QRDR mutation plays a marked role in mediating fluoroquinolone resistance.