Yeast cells need to respond to a variety of stresses found in such different conditions as gastrointestinal tract after probiotic ingestion or fermentation vat during ethanol production. In the present study, H+ neutralisation capacity, membrane fatty acid composition, H+-ATPase activity, and cytosolic Ca2+ concentration were evaluated in yeast cells used for probiotic (Saccharomyces boulardii) and laboratory (Saccharomyces cerevisiae W303) purposes, as well as in some W303 mutant strains for ENA1 gene and S. cerevisiae BY4741. Results show that the H+ internal concentration of yeast is regulated by several systems, including the plasma membrane H+-ATPase, and that Ena1p has an important but undefined role in the cellular response to acid. Membrane fatty acid composition of S. cerevisiae W303 strain was affected by exposure to acidic pH, but the presence of 86？mM NaCl prevented this effect, whereas membrane fatty acid composition of S. boulardii was unaffected by acidic pH. We also demonstrated that the acid stress response is dependent on calcium metabolism and blocked by FK 506. 1. Introduction To survive and proliferate, free-living organisms must adapt to changes in their environment. Exposure of the Saccharomyces cerevisiae to environmental stresses, such as toxic ions [1, 2], ethanol [3], or changes in temperature [4] or pH [5], triggers biochemical and gene expression changes [6]. Yeast rapid exposure to inorganic acids is of interest, because such exposure occurs under environmental (e.g., yeast probiotics passing through the gastrointestinal system) and industrial conditions (e.g., sulphuric acid to eliminate bacterial contamination in yeast cultures that are to be reused for fermentation [7]). Saccharomyces cerevisiae grows well over a wide range of pH but grows better in acidic than in alkaline pH [5, 8]. Studies have demonstrated changes in the expression of hundred genes in S. cerevisiae following alterations in pH [9–11]. The responses of S. cerevisiae to alkaline pH have been reviewed by Ari？o [5] and involve various signalling pathways. In particular, the role of calcineurin on alkaline stress was suggested early on, and the involvement of calcium signalling in this response was reported in subsequent works [5, 10, 12, 13]. Responses to alkaline pH have also been described for Candida albicans [14] and Aspergillus nidulans [15, 16]. Responses to acid stress have been studied in yeast cells that were artificially exposed to weak organic acids [9, 17], food preservatives [17, 18], and herbicides [19]. In response to exposure to weak acids,
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