The antimicrobial systems of Streptococcus suis promote niche competition in pig tonsils

Streptococcus suis can cause severe infections in pigs and humans. The tonsils of pigs are major niches for S. suis, and different serotypes of S. suis can be found in the same tonsil. Pig tonsil colonization by S. suis is believed to be an important source of infection for humans and pigs. However, how S. suis competes for a stable tonsil niche is unknown. Here, we found that S. suis 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 in vivo. 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 S. suis. Antibacterial activity analysis and 16S rRNA gene sequencing of the culture-independent and culture-dependent pig tonsillar microbiome revealed that Lcn351 mainly targets S. suis, one of the core microbiomes in pig tonsils. Taken together, our results revealed the mechanism of the stable persistence of S. suis in the tonsil niche, which might have important implications for S. suis epidemiology, potentially influencing strain prevalence and disease progression.