Abstract
Background:
Recently, the misuse of antibiotics has led to the rapid evolution of bacterial resistance, with bacteria of animal origin increasingly recognized as reservoirs of resistance genes. The atypical streptococcal species Streptococcus hyovaginalis, originally isolated from porcine vaginal specimens, has been increasingly identified in respiratory tract samples. Its recovery from diseased livestock suggests a broader clinical relevance than previously understood. This expanded host and tissue range necessitates a reassessment of its antibiotic resistance profiles. Its pathogenicity remains unclear, and its resistance profile has not been systematically studied. Given its extensive multidrug resistance, which poses a potential risk for the spread of resistance genes, and its ability to transmit across species, which may breach host barriers, in-depth research on this bacterium is particularly urgent.

Aim:
This study aims to systematically reveal the multidrug-resistant phenotypes of S. hyovaginalis strains in swine in Jilin Province, analyze their resistance gene repertoire, and explore their population genetic diversity through phylogenetic analysis.

Methods:
A total of 16 strains isolated from nasal swabs of 213 sick piglets between March and July 2023 were identified as S. hyovaginalis based on 16S rRNA gene sequencing. These isolates were subjected to antimicrobial susceptibility testing using the agar dilution method to determine minimum inhibitory concentrations against 21 antimicrobial agents. The study first conducted second-generation sequencing analysis and then comprehensively screened for resistance genes by searching the Comprehensive Antibiotic Resistance Database up to January 2025.

Results:
These isolates exhibited multidrug resistance phenotypes, demonstrating antibiotic resistance spanning resistance to antibiotics spanning multiple drug classes. High resistance rates were observed for β-lactam antibiotics: ampicillin (100% of isolates) and lincomycin (94%). Genomic analysis using second-generation sequencing revealed 31 distinct antimicrobial resistance genes among the isolates. Functional annotation identified five genes, such as lsa (E) and lnu (B), associated with lincosamide resistance and four genes, such as patB, conferring aminoglycoside resistance mechanisms.

Conclusion:
This study provides the first systematic characterization of MD Rmultidrug resistance in porcine S. hyovaginalis from Jilin Province, revealing a complex resistance gene composition. These findings establish a critical foundation for future mechanistic studies and underscore the potential role of this bacterium as a reservoir for disseminating resistance genes.

Key words: Streptococcus hyovaginalis; Pig; Resistance genes; Second-generation sequencing analysis.