Genetic diversity, virulence genes, and antimicrobial resistance of Streptococcus dysgalactiae isolates from different aquatic animal sources

Lancefield group C Streptococcus dysgalactiae represents an etiological agent causing high mortality in many farmed fish species worldwide. Despite its pathogenic importance, limited epidemiological knowledge related to this pathogen is available. In the present study, 79 S. dysgalactiae isolates from diseased aquatic animals in Taiwan (n = 67) and Japan (n = 12) were characterized using pulsed-field gel electrophoresis (PFGE). The distribution of isolate virulence factors and antimicrobial susceptibility were also investigated. PFGE with SmaI and ApaI digestions displayed 19 and 20 different pulsotypes respectively, reflecting a genetic diversity among isolates from different sources. All examined strains harbored the virulence-associated genes sagA, NAPlr, and alpha-enolase, whereas 76/79 (96.2%) and 77/79 (97.5%) harbored spegg and sof, respectively. PCR analysis of the most common tetracycline and macrolide resistance genes demonstrated that isolates carried tet(M) (14/79, 17.7%), tet(S) (12/79, 15.2%), erm(B) (5/79, 6.3%), and mef(A) and msr(D) (3/79, 3.8%). Isolates harboring one or more resistance genes showed resistance to drugs by the disk diffusion method. Together, our data indicates that S. dysgalactiae isolates in Taiwan and Japan exhibit distinct genomic profiles associated with their host species and geographical origins, harbor multiple putative virulence factors, and some are found resistant to tetracycline and erythromycin.