Isolation and characterization of Streptococcus suis strains from swine in Jiangsu province, China

The aim of this study is to investigate Streptococcus suis strains present in Jiangsu province, China.In all, 1650 nasal and anal swab samples and 100 tonsils were collected from clinically healthy swine. Culture characteristics, colony morphology and PCR-based analysis of the glutamate dehydrogenase (gdh) gene were performed for S. suis identification, and eight isolates were confirmed to be S. suis. The isolates serogroups were identified by agglutinating test, including 4, 7, 3, 5 and 8. Gene profiling by PCR showed that the manN, purD, orf2, gdh genes were conserved among the isolates and that 50% of the isolates carried dltA, pgdA, srtA and sspA. Antimicrobial susceptibility test showed the isolates displayed resistance to clindamycin, erythromycin, tetracycline, penicillin G, vancomycin and linezolid; while none was resistant to chloramphenicol, multi-drug resistance was seen in most of the isolates. Finally, the LD50 (assessed by zebrafish) of isolates RD105 was 2·6431 × 105 and HA24 was 7·1198 × 106 , which showed RD105 more virulent and was consistent with the results of virulence factor identification.There is a very low proportion of S. suis in the healthy pigs. The virulence factors were related to pathogenicity. Bacteria in Nantong possess greater virulence potential than those in Huaian.Systematic investigation of S. suis provided the most basic theoretical support for the prevention and control of swine streptococcosis.