Novel Quinolone Nonsusceptible Streptococcus canis Strains with Point Mutations in Quinolone Resistance-determining Regions and Their Related Factors

This study investigated quinolone nonsusceptible Streptococcus canis with point mutations in quinolone resistance-determining regions (QRDRs). After selecting targets from 185 isolates, we conducted antimicrobial susceptibility testing using levofloxacin, ciprofloxacin, norfloxacin, and moxifloxacin. We also determined amino acid sequences of QRDRs in gyrA/ gyrB/parC/parE genes and their point mutations. Finally, we performed S. canis-derived M-like protein (SCM) allele typing, multilocus sequence typing, and antimicrobial resistance genotyping. Correlations between nonsusceptible strains and their related factors were examined. We found 13 (7.0%) nonsusceptible isolates consisting of two classes, high-level minimum inhibitory concentrations (MICs) (n = 7, 3.8%), and low-level MICs (n = 6, 3.2%). The mutations Ser81Phe/Ser81Tyr/Glu85Lys in gyrA, Ser67Phe/Ser67Tyr/Asp71Tyr in parC, Asp438Asn in parE, and Gly408Asp in gyrB were observed in these nonsusceptible strains. The common mutations were Ser81 and Ser67/Asp71, whereas we found one strain each with Glu85, Asp438 and Gly408 mutations. There was a significant correlation between the nonsusceptible isolates and presence of SCM allele type 2, sequence type 46, tetracycline-resistance genes, and macrolide/lincosamide-resistance genes. These results could be used in the future by veterinarians while treating companion animals with clinical symptoms of streptococcal infections.