The antimicrobial activity of citric acid (CA) is often evaluated without pH adjustment or control and its impact on microorganisms is better understood in acidic conditions. However, the biocidal action of the fully ionized CA molecule, predominantly available at higher pH, has not been previously investigated. The objective of this study was to evaluate the antimicrobial effect of high (10%) and low (1%) concentrations of CA, each adjusted over a wide range of pH values (4.5, 6.5, and 9.5) relative to the controls exposed to corresponding pH levels alone (no CA). The viability and morphology of Escherichia coli and Klebsiella aerogenes were evaluated by using a culture-based enumeration assay in parallel with direct SEM imaging. Overall, the highest membrane damage and loss in viability was achieved with 10% CA at pH 9.5, which yielded at least 4.6 log10 CFU.mL-1 (P<0.001) reductions in both organisms. Insight into the superior efficacy of CA at high pH is proposed based on zeta potential measurements which reveal a more negatively charged bacterial surface at higher pH. This pH-dependent increase of surface charge may have rendered the cells potentially more sensitive towards chelants such as CA3- that interact with membrane-stabilizing divalent metals.

中文翻译：

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pH值对柠檬酸对革兰氏阴性菌抗菌活性的影响

柠檬酸 (CA) 的抗菌活性通常在没有调节或控制 pH 值的情况下进行评估，并且在酸性条件下可以更好地了解其对微生物的影响。然而，完全电离的 CA 分子的杀生物作用，主要在较高的 pH 值下可用，以前尚未研究过。本研究的目的是评估高 (10%) 和低 (1%) 浓度的 CA 的抗菌作用，相对于暴露于暴露的对照，每种都在较宽的 pH 值范围（4.5、6.5 和 9.5）内进行调整相应的 pH 值水平（无 CA）。大肠杆菌和产气克雷伯菌的生存力和形态通过使用基于培养的计数测定与直接 SEM 成像平行进行评估。总体而言，在 pH 9.5 条件下使用 10% CA 实现了最高的膜损伤和活力损失，这使两种生物体至少减少了 4.6 log10 CFU.mL-1 (P<0.001)。基于 zeta 电位测量提出了对 CA 在高 pH 下的优越功效的洞察，该测量揭示了在较高 pH 下带更多负电荷的细菌表面。这种依赖于 pH 值的表面电荷增加可能使细胞对螯合剂（如 CA3-）更敏感，这些螯合剂与膜稳定二价金属相互作用。