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Antimicrobial Resistance, Serotypes, Virulence Gene Profiles, and Molecular Characterization of Streptococcus suis Isolated from Healthy Pigs in Thailand

Phirabhat Saengsawang, Pakpoom Tadee, Prapas Patchanee, Watcharapong Mitsuwan and 5 more

Antibiotics | Jul 3, 2026

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It is demonstrated that healthy pigs can harbor genetically diverse and antimicrobial resistant S. suis, highlighting the importance of continued surveillance to support antimicrobial stewardship and reduce zoonotic risk.

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Streptococcus suis is an important cause of disease in pigs and an emerging source of severe human infection, with increasing concerns regarding antimicrobial resistance. This study assessed the occurrence and seroprevalence of S. suis in healthy pigs and characterized the antimicrobial susceptibility, serotypes, virulence associated genes, and genomic features of colonizing isolates. A total of 58 pigs (13 farm pigs and 45 abattoir pigs) were sampled. Among 176 presumptive isolates recovered by culture, 60 were confirmed as S. suis by polymerase chain reaction (PCR) and were subsequently analyzed. Antimicrobial susceptibility was determined by the disc diffusion method, selected resistance and virulence genes were detected by conventional PCR, and two representative isolates were subjected to whole-genome sequencing (WGS) and comparative genomic analyses. Seroprevalence was determined using an indirect ELISA detecting IgG antibodies against S. suis serotype 2 antigen. S. suis was confirmed in 60/176 presumptive isolates (34.1%), most frequently from nasal swabs and tonsils, whereas no blood isolates were detected. The seropositivity rate was 65.5%. High resistance rates were observed for ceftriaxone, ampicillin, cefepime, clindamycin, and tetracycline, and multidrug resistance was common. Tetracycline- and macrolide-associated resistance genes were frequently detected. Serotypes 2/1⁄2 predominated, whereas major classical invasive virulence genes were not detected, consistent with predominantly colonizing strains. Whole genome sequencing of two representative isolates confirmed species identity and revealed substantial genomic diversity, including a markedly larger genome in a tonsillar isolate, suggesting acquisition of accessory genetic elements. These findings demonstrate that healthy pigs can harbor genetically diverse and antimicrobial resistant S. suis, highlighting the importance of continued surveillance to support antimicrobial stewardship and reduce zoonotic risk.

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Phirabhat Saengsawang

first | Kasetsart University | ORCID 0000-0002-0432-0998

Pakpoom Tadee

middle | Chiang Mai University | ORCID 0000-0003-3171-9678

Prapas Patchanee

middle | Chiang Mai University

Watcharapong Mitsuwan

middle | Walailak University | ORCID 0000-0001-9048-3955

Sumalee Boonmar

middle | Walailak University | ORCID 0000-0002-5616-2717

Hidenori Kabeya

middle | Nihon University | ORCID 0000-0002-4127-6287

Nattinee Kittiwan

middle | ORCID 0000-0003-0601-1467

Ravisa Warin

middle | Chiang Mai University

Ratchadaporn Boripun

last | Walailak University | ORCID 0000-0001-5786-2769

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BibTeX

@article{Saengsawang2026Antimicrobial,
  title = {Antimicrobial Resistance, Serotypes, Virulence Gene Profiles, and Molecular Characterization of Streptococcus suis Isolated from Healthy Pigs in Thailand},
  author = {Phirabhat Saengsawang and Pakpoom Tadee and Prapas Patchanee and Watcharapong Mitsuwan and Sumalee Boonmar and Hidenori Kabeya and Nattinee Kittiwan and Ravisa Warin and Ratchadaporn Boripun},
  journal = {Antibiotics},
  year = {2026},
  doi = {10.3390/antibiotics15070660},
  url = {https://doi.org/10.3390/antibiotics15070660}
}

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