Investigating the pH dependent antifungal effects of butyrate on Candida albicans.

INTRODUCTION: Candida species are common members of the human gastrointestinal microbiome but are also associated with a range of diseases when microbial community balance is disrupted. Short-chain fatty acids produced by gut bacteria, particularly butyrate, play important roles in host-microbiome interactions and are increasingly explored as therapeutic modulators of microbial composition. Butyrate is present both as a microbial metabolite and dietary component and has been reported to influence Candida abundance within the gut. However, the antifungal activity of sodium butyrate (NaB) against Candida albicans under physiologically relevant gut pH conditions remains poorly understood. METHODS: We examined the effects of NaB on C. albicans growth and physiology under pH conditions representative of the gastrointestinal environment. Growth, hyphal transition, respiration and biofilm formation were assessed in the presence of NaB at neutral pH. Parallel experiments at acidic pH (pH 4.0) evaluated fungicidal activity and associated cellular responses, including mitochondrial membrane potential, reactive oxygen species (ROS) accumulation, and intracellular calcium homeostasis. Histone deacetylase inhibitory activity of NaB was also assessed to determine its early cellular effects. RESULTS: NaB displayed rapid histone deacetylase inhibitor activity in C. albicans and significantly inhibited growth, hyphal morphogenesis, respiration, and biofilm formation at neutral pH. In contrast, under acidic conditions (pH 4.0) NaB exhibited fungicidal activity. This lethal effect was associated with mitochondrial depolarisation, elevated ROS levels, and disruption of intracellular calcium regulation. Further analyses indicated that oxidative stress and loss of calcium homeostasis are key contributors to NaB-induced cell death under acidic conditions. DISCUSSION: These findings reveal a strong pH dependence in the antifungal activity of butyrate against C. albicans. While NaB acts primarily as a physiological inhibitor of growth and virulence traits at neutral pH, acidic conditions convert its activity to a fungicidal mechanism driven by mitochondrial dysfunction, oxidative stress, and calcium dysregulation. This pH-dependent behaviour has implications for understanding microbiome-derived metabolites in fungal ecology within the gut and highlights the potential of butyrate-based strategies to modulate C. albicans overgrowth.