The Genomic Characteristics of Potential Probiotics: Two Streptococcus salivarius Isolates from a Healthy Individual in China

The isolation and characterization of novel probiotics from dairy products, fermented foods, and the gut have gained significant attention. In particular, <i>Streptococcus salivarius</i> shows promise for use in oral probiotic preparations. In this study, we isolated two strains of <i>S. salivarius</i>-S.82.15 and S.82.20-from the oral cavity of a healthy individual. These strains exhibited distinct antimicrobial profiles. We thoroughly assessed the morphology and growth patterns of both strains and confirmed auto-aggregation and hemolytic activity. Through comprehensive genomic analysis, we found notable strain differences within the same bacterial species isolated from the same individual. Notably, the presence or absence of plasmids varied between the two strains. The genome of S.82.15 spans 2,175,688 bps and contains 1994 coding DNA sequences (CDSs), while S.82.20 has a genome size of 2,414,610 bps, a GC content of 40.62%, and 2276 annotated CDSs. Both strains demonstrated antibacterial activity against Group A <i>Streptococcus</i> (GAS), <i>Micrococcus. luteus</i>, and <i>Porphyromonas gingivalis</i>. To investigate the antibacterial properties further, we identified a gene cluster of salivaricin 9 on the plasmid of S.82.20 and a <i>blp</i> gene family on the chromosomes of both S.82.15 and S.82.20. Moreover, the gene expression of the <i>blp</i> family was upregulated when the isolated strains were co-cultured with GAS.