Researchers found that many common oral bacteria, especially Streptococcus mitis/oralis, can calm down inflammation in mouth cells by blocking a key signal (NF‑κB) and reducing a warning chemical (CXCL8). This suggests that a healthy mix of these bacteria might help keep the mouth and possibly the whole body less inflamed.

<b>Introduction.</b> Oral tissues are generally homeostatic despite exposure to many potential inflammatory agents including the resident microbiota. This requires the balancing of inflammation by regulatory mechanisms and/or anti-inflammatory commensal bacteria. Thus, the levels of anti-inflammatory commensal bacteria in resident populations may be critical in maintaining this homeostatic balance.<b>Hypothesis/Gap Statement.</b> The incidence of immunosuppressive streptococci in the oral cavity is not well established. Determining the proportion of these organisms and the mechanisms involved may help to understand host-microbe homeostasis and inform development of probiotics or prebiotics in the maintenance of oral health.<b>Aim.</b> To determine the incidence and potential modes of action of immunosuppressive capacity in resident oral streptococci.<b>Methodology.</b> Supragingival plaque was collected from five healthy participants and supragingival and subgingival plaque from five with gingivitis. Twenty streptococci from each sample were co-cultured with epithelial cells&#xb1;flagellin&#x2009;or LL-37. CXCL8 secretion was detected by ELISA, induction of cytotoxicity in human epithelial cells by lactate dehydrogenase release and NF&#x3ba;B-activation using a reporter cell line. Bacterial identification was achieved through partial 16S rRNA gene sequencing and next-generation sequencing.<b>Results.</b> CXCL8 secretion was inhibited by 94/300 isolates. Immunosuppressive isolates were detected in supragingival plaque from healthy (4/5) and gingivitis (4/5) samples, and in 2/5 subgingival (gingivitis) plaque samples. Most were <i>Streptococcus mitis/oralis</i>. Seventeen representative immunosuppressive isolates all inhibited NF&#x3ba;B activation. The immunosuppressive mechanism was strain specific, often mediated by ultra-violet light-labile factors, whilst bacterial viability was essential in certain species.<b>Conclusion.</b> Many streptococci isolated from plaque suppressed epithelial cell CXCL8 secretion, via inhibition of NF&#x3ba;B. This phenomenon may play an important role in oral host-microbe homeostasis.