Environmental pH controls antimicrobial production by human probiotic <i>Streptococcus salivarius</i>

Probiotic bacteria are important tools in combating bacterial infections. Probiotics exert their antimicrobial activity via several mechanisms, including antimicrobial production. However, discrepancies exist between the <i>in vitro</i> and <i>in vivo</i> efficacies of probiotics in inhibiting pathogen growth. Understanding the host and environmental factors that influence antimicrobial production and activity is critical for improving probiotic efficacy. In this study, we showed that the antimicrobial salivabactin produced by human oral probiotic <i>Streptococcus salivarius</i> K12 is active at acidic pH. We further elucidated the molecular mechanism by which <i>S. salivarius</i> coordinates salivabactin production in concert with environmental acidification, thereby maximizing salivabactin antimicrobial activity.