Genomic Characterization of Potential Opportunistic Zoonotic Streptococcus parasuis Isolated in China

(1) Background: <i>S. parasuis</i> is a potential opportunistic zoonotic pathogen that can infect pigs, cattle, and humans, composed of former members of <i>S. suis</i> serotypes 20, 22, and 26. In recent years, unclassified serotypes and a serotype 11 <i>S. parasuis</i> have been discovered. (2) Methods: We characterized two <i>S. parasuis</i> strains (FZ1 and FZ2) isolated from brain samples of paralyzed pigs and examined evolutionary divergence among 22 available <i>S. parasuis</i> and 8 serotype 2 <i>S. suis</i> genomes through whole-genome sequencing and comparative genomic analysis. We compared virulence genes (VGs) and antibiotic resistance genes (ARGs) and analyzed mobile genetic elements (MGEs) in FZ1 and FZ2. (3) Results: Comparative genomics revealed that <i>srtC</i>, <i>ctpV</i>, and <i>sugC</i> may represent key virulence determinants in <i>S. parasuis</i>, although their pathogenic potential appears attenuated compared to serotype 2 <i>S. suis</i>. In addition, <i>S. parasuis</i> exhibited primary resistance to aminoglycosides, macrolides, tetracyclines, and oxazolidinones, while demonstrating heightened susceptibility to oxazolidinone-class antibiotics. Moreover, we found an important association between MGEs and antibiotic resistance in <i>S. parasuis</i> FZ1 and FZ2. (4) Conclusions: This study provides new insights into the genomic and evolutionary characteristics of <i>S. parasuis</i> and provides a new basis for the study of bacterial pathogenesis and drug resistance in the future.