Table 1_Investigating the pH dependent antifungal effects of butyrate on Candida albicans.docx

IntroductionCandida 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.
MethodsWe 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.
ResultsNaB 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.
DiscussionThese 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.

引言
念珠菌属（Candida）物种是人类胃肠道微生物组的常见组成成员，但当微生物群落平衡被打破时，它们也与多种疾病相关联。肠道细菌产生的短链脂肪酸，尤其是丁酸，在宿主与微生物组的互作过程中发挥重要作用，且其作为调控微生物组成的治疗性调节剂的研究日益增多。丁酸既可以作为微生物代谢产物存在，也可作为膳食成分存在，已有研究表明其可影响肠道内念珠菌的丰度。然而，丁酸钠（NaB）对白色念珠菌（Candida albicans）在生理相关肠道pH条件下的抗真菌活性，目前仍鲜为人知。

研究方法
本研究在模拟胃肠道环境的pH条件下，探究了丁酸钠对白色念珠菌生长及生理状态的影响。在中性pH条件下，我们评估了丁酸钠存在时白色念珠菌的生长、菌丝形态转换、呼吸作用及生物被膜形成能力。在pH 4.0的酸性条件下开展平行实验，以评估丁酸钠的杀真菌活性及其相关细胞应答，包括线粒体膜电位、活性氧（ROS）积累以及细胞内钙稳态。本研究还评估了丁酸钠的组蛋白去乙酰化酶（Histone deacetylase）抑制活性，以明确其早期细胞作用机制。

研究结果
丁酸钠在白色念珠菌中表现出快速的组蛋白去乙酰化酶抑制活性，且在中性pH条件下可显著抑制其生长、菌丝形态发生、呼吸作用及生物被膜形成。与之相反，在pH 4.0的酸性条件下，丁酸钠则表现出杀真菌活性。这种致死效应与线粒体去极化、ROS水平升高以及细胞内钙调节紊乱相关。进一步分析表明，氧化应激与钙稳态失衡是酸性条件下丁酸钠诱导细胞死亡的关键诱因。

讨论
本研究结果表明，丁酸对白色念珠菌的抗真菌活性具有显著的pH依赖性。尽管在中性pH条件下，丁酸钠主要作为生长与毒力性状的生理抑制剂发挥作用，但酸性条件可将其活性转化为以线粒体功能障碍、氧化应激及钙调节紊乱为驱动机制的杀真菌途径。这种pH依赖的行为特性，有助于理解肠道真菌生态中微生物组来源的代谢产物，并凸显了基于丁酸的策略在调控白色念珠菌过度增殖方面的应用潜力。