Antimicrobial Resistance and Genomic Characterization of a Novel ST181 Streptococcus parasuis Clinical Isolate from Human Pleural Fluid

Background/Objectives: Streptococcus parasuis is an under-recognized member of the Streptococcus suis complex that can be misidentified by routine diagnostic methods. Human clinical isolates remain sparsely characterized. This study investigated the antimicrobial phenotype, genomic features, population position, and larval pathogenicity of a human pleural-fluid isolate. Methods: Strain HUGSPH was recovered during the care of a patient with pneumonia. Hybrid whole-genome sequencing (WGS), average nucleotide identity analysis, multilocus sequence typing (MLST), core-genome phylogenetics, comparative pan-genomics, and an exploratory host-origin association analysis were performed. Minimum inhibitory concentrations (MICs) were determined using the VITEK 2 Compact system with an AST-ST03 card, and virulence was evaluated in Galleria mellonella larvae. Results: Genome-based analysis confirmed HUGSPH as S. parasuis and assigned it to the novel sequence type (ST) 181. The isolate clustered within a predominantly human-origin phylogenetic clade. Because species-specific clinical breakpoints are unavailable, the MICs were interpreted provisionally using explicitly stated surrogate Clinical and Laboratory Standards Institute (CLSI) criteria; erythromycin, clindamycin, and levofloxacin were categorized as resistant under those criteria. Several predicted antimicrobial-resistance-associated genes were detected. In a restricted comparison of seven human-origin and nine swine-origin genomes, eight genes were present in all included human-origin isolates and absent from all included swine-origin isolates, whereas metQ, metP, and metN showed the reciprocal pattern. HUGSPH caused dose-dependent larval mortality, reaching 100% at 107 colony-forming units (CFU) by 96 h. Conclusions: HUGSPH expands the genomic record of human clinical S. parasuis and highlights the diagnostic and surveillance relevance of antimicrobial resistance in this species. The susceptibility categories, source-associated genes, and larval phenotype require validation by reference susceptibility testing, broader phylogenetically balanced collections, comparative strains, and mammalian models.