Human Saliva Induces Acetoin-Dependent Metabolic Adaptation that Promotes Stress Tolerance in Streptococcus mutans

Growth of Streptococcus mutans in chemically defined medium supplemented with pooled human saliva induces acetoin dehydrogenase operon expression and boosts oxidative stress survival by ~10,000-fold.

• Why it matters: Saliva is a constant host-derived exposure for oral microbes, but how it remodels metabolism and stress adaptation in S. mutans has been incompletely defined.
• What they did: Using RNA-Seq, targeted metabolite measurements, gene deletions, and Tn-Seq, the study quantified extracellular acetoin and tested survival after hydrogen peroxide, including acetoin-pathway mutants.
• The result: Saliva increased extracellular acetoin across all S. mutans isolates tested (but not in two commensal streptococci), and saliva-grown cells showed ~10,000-fold higher hydrogen peroxide survival; acetoin-pathway mutants retained protection but had ~100-fold lower survival than wild type.