Streptococcus salivarius is a naturally occurring oral probiotic bacterium that has been investigated for its potential to reduce halitosis (bad breath). Several clinical studies have evaluated the use of specific strains, particularly S. salivarius K12 and M18, as oral probiotics for halitosis management. The rationale is that these strains can colonize the oral cavity, especially the tongue and pharynx, and compete with or inhibit growth of bacteria associated with volatile sulfur compound (VSC) production—compounds responsible for malodor.

Randomized controlled trials have demonstrated that administration of S. salivarius K12 can reduce VSC levels and improve breath scores in both healthy individuals and those with oral malodor. For example, a 2006 double-blind, placebo-controlled study by Burton et al. showed a significant reduction in halitosis following 3 days of K12 lozenge use. The mechanism is believed to involve bacteriocin production, which suppresses halitosis-causing bacteria such as Solobacterium moorei and Fusobacterium nucleatum.

While results are promising, the evidence base is still moderate, with relatively small sample sizes and short durations in existing trials. Longer-term and larger scale studies are needed to confirm efficacy and determine optimal dosing. Nonetheless, S. salivarius has a plausible mechanism of action and some clinical validation for supporting halitosis reduction.