UNLABELLED

K12 (SAL) is an oral probiotic used to treat or prevent oral infections caused by human pathogens. SAL produces at least three antimicrobials to exert its antimicrobial activity, namely, salivaricin A and salivaricin B, and the newly identified salivabactin. Salivabactin production is catalyzed by a polyketide/non-ribosomal peptide synthase hybrid biosynthetic gene cluster (BGC), termed as . The expression and salivabactin production are transient during SAL growth and , which may negatively impact SAL probiotic efficacy. To understand the molecular basis for transient expression, we assessed the impact of environmental pH on expression. We found that environmental acidification is a critical factor in promoting salivabactin antimicrobial activity and production by inducing expression. We further showed that acidic pH directly influences the quorum-sensing system that controls expression. During environmental acidification, SAL cytosol is acidified, which is sensed by a pH-sensitive histidine switch in the cytosolic transcription regulator, NrpR. The protonation of histidine during cytosolic acidification promotes high-affinity interactions between NrpR and its cognate intercellular signaling peptide, NIP, which leads to upregulation of expression. Collectively, our results indicate that SAL uses a sophisticated regulatory mechanism to orchestrate salivabactin production in an environment that is conducive to its antimicrobial activity.

IMPORTANCE

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 and 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 K12 is active at acidic pH. We further elucidated the molecular mechanism by which coordinates salivabactin production in concert with environmental acidification, thereby maximizing salivabactin antimicrobial activity.