Molecular epidemiology and antimicrobial resistance of human Streptococcus suis isolates in Guangxi, China, 2015–2021

Abstract Background Streptococcus suis is an important zoonotic pathogen that poses a substantial threat to both public health and the swine industry. Multiple human infection cases have been reported in China, including large-scale outbreaks associated with highly virulent strains. However, molecular epidemiological data from Guangxi province remain limited. This study aimed to investigate the antimicrobial resistance patterns and molecular epidemiological characteristics of human-derived S. suis isolates collected in Guangxi, China. Methods A total of 41 clinical isolates initially identified as S. suis were collected from human infections in Baise City, Guangxi, between 2015 and 2021. Antimicrobial susceptibility testing was performed according to Clinical and Laboratory Standards Institute (CLSI) M100 guidelines (2023 edtion). Whole-genome sequencing was conducted to determine serotypes, multilocus sequence types (MLST), antimicrobial resistance genes, and virulence-associated genes using bioinformatic analyses based on the VFDB and ResFinder databases. Phylogenetic relationships among the isolates were inferred from MLST data. Results Among the 41 isolates, 39 were confirmed as S. suis , whereas two isolates were reclassified as Streptococcus parasuis by genomic analysis. Most infections occurred in middle-aged and elderly men, and cerebrospinal fluid was the predominant specimen source. Antimicrobial susceptibility testing showed high resistance rates to tetracycline (92.7%), erythromycin (62.5%), and clindamycin (62.5%), while all isolates remained susceptible to β-lactams, vancomycin, linezolid, levofloxacin, and meropenem. Serotype analysis revealed that serotype 2 (68.29%) was the predominant serotype, followed by serotype 1 (26.83%). MLST analysis identified ST1 (70.73%) as the dominant sequence type, whereas ST7 represented the second most common lineage. Genomic analysis detected multiple antimicrobial resistance genes, most frequently ant (6)-Ia , erm(B) , tet (M) , and tet (O) . Notably, the tet (40) gene was detected exclusively in ST7 isolates. Most isolates carried multiple virulence-associated genes, whereas the two S. parasuis isolates harbored substantially fewer virulence determinants. Conclusions Serotype 2 and ST1 are predominant among human-derived S. suis isolates in Baise City, Guangxi. The isolates exhibit widespread multidrug resistance and frequently carry genes associated with tetracycline and macrolide resistance. These findings underscore the importance of continued regional surveillance of the molecular epidemiology and antimicrobial resistance of pathogenic S. suis to support early warning and prevention of future outbreaks.