Abstract: The pathogenic bacteria was isolated from nursery piglets exhibiting acute fatal symptoms of streptococcosis at a pig farm in Hubei Province. The pathogen was identified through bacterial culture, morphological observation, biochemical tests, and molecular analysis. After inoculating clinical samples onto TSA medium supplemented with 5% sheep blood, Gram-positive cocci arranged in chains were isolated. PCR amplification using serotype 9-specific primers and subsequent sequencing confirmed the isolate as Streptococcus suis serotype 9, designated strain XF-1. Virulence gene screening revealed that XF-1 carried fbps, orf, sly, and gapdh genes, while epf and mrp genes were negative. Antimicrobial susceptibility testing indicated intermediate sensitivity to cefuroxime, ceftiofur, and ampicillin, but resistance to 12 antibiotics including cephalothin and gentamicin. Mouse pathogenicity trials demonstrated high virulence, all mice challenged with ≥; 5×; 10⁸ CFU died within 7 days (groups 20×; 10⁸, 10×; 10⁸, and 5×; 10⁸ CFU), while the 1×; 10⁸ CFU group showed 40% mortality. Bacteria re-isolated from deceased mouse organs were PCR-confirmed as identical to XF-1.

Key words: Streptococcus suis serotype 9, isolation and identification, virulence gene, pathogenicity, antimicrobial susceptibility test