The antimicrobial systems of <i>Streptococcus suis</i> promote niche competition in pig tonsils

<i>Streptococcus suis</i> can cause severe infections in pigs and humans. The tonsils of pigs are major niches for <i>S. suis</i>, and different serotypes of <i>S. suis</i> can be found in the same tonsil. Pig tonsil colonization by <i>S. suis</i> is believed to be an important source of infection for humans and pigs. However, how <i>S. suis</i> competes for a stable tonsil niche is unknown. Here, we found that <i>S. suis</i> strain WUSS351, isolated from a healthy pig tonsil, is virulent and multidrug-resistant. The ABC transporter system SstFEG, conferring resistance to bacitracin, was reported to confer a competitive survival advantage <i>in vivo</i>. In addition, strain WUSS351 has several antimicrobial systems, including a novel type VII secretion system (T7SS), lantibiotic bacteriocin, and lactococcin972-like bacteriocin Lcn351. Bacterial competition experiments demonstrated T7SS-mediated cell contact-dependent antagonism of <i>S. suis</i>. Antibacterial activity analysis and 16S rRNA gene sequencing of the culture-independent and culture-dependent pig tonsillar microbiome revealed that Lcn351 mainly targets <i>S. suis</i>, one of the core microbiomes in pig tonsils. Taken together, our results revealed the mechanism of the stable persistence of <i>S. suis</i> in the tonsil niche, which might have important implications for <i>S. suis</i> epidemiology, potentially influencing strain prevalence and disease progression.