Environmental pH controls antimicrobial production by human probiotic Streptococcus salivarius

Streptococcus salivarius 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 sar-BGC. The sar-BGC expression and salivabactin production are transient during SAL growth in vitro and in vivo, which may negatively impact SAL probiotic efficacy. To understand the molecular basis for transient sar-BGC expression, we assessed the impact of environmental pH on sar-BGC expression. We found that environmental acidification is a critical factor in promoting salivabactin antimicrobial activity and production by inducing sar-BGC expression. We further showed that acidic pH directly influences the quorum-sensing system that controls sar-BGC 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 sar-BGC 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.