Identification and characterization of Streptococcus suis strains isolated from eastern China Swine Farms, 2021-2023

The Streptococcus suis (S. suis) is an important zoonotic pathogen that causes streptococcal disease in pigs and poses a threat to humans. This study provides an understanding of the prevalence of  S.suis in eastern China and provides guidance for clinical prophylaxis. From 2021 to 2023, a total of 143 strains of S. suis were isolated from 1642 lung tissue and nasal swabs from healthy and suspected infected pigs in Shandong Province, China, using the Phenotypic tests and PCR technique. The isolates were then tested for serotype, virulence-related genes, and resistance genes. Among the 143 isolates, type 2 was the predominant serotype with 98 isolates (98/143, 68.5%), followed by type 5 with 22 isolates (22/143, 15.3%), type 4 with 6 isolates (6/143, 4.2%), type 19 with 4 isolates (4/143, 2.8%) and type 21 with 5 isolates (5/143, 3.5%), respectively. A minimum of 78.3% of the strains exhibited the presence of virulence-related genes including pgda, dlta, mann, fbps, orf2, and sspa, whereas the virulence-associated genes Sum, Sly, and Salkr are not widely prevalent. For the detection of resistance genes, it was found that the tetO gene had a high detection rate of 70.1% (101/143), whereas neither the pbp2b gene nor the cat1 and cat2 genes were detected. Antimicrobial susceptibility testing revealed that 96.5% (138/143) of the isolates exhibited multidrug resistance (MDR). And polypeptide B was found to be tolerated by 125 of the 143 strains (87.4%). Although we did not detect the β-lactam resistance gene in any of the 143 strains, an average of 39.2% of the strains were resistant to β-lactam antibiotics. The results of the current study is thought it may be help to understand the prevalence of S. suis and provide important insights into treatment and prevention.