Photocatalysis on yeast cells: Toward targets and mechanisms

We have investigated the antimicrobial effects of photocatalysis on yeast (Saccharomyces cerevisiae), an essential eukaryotic unicellular model of living cells. As compared to UV-A irradiation, photocatalytic inactivation kinetics revealed a faster microbial cell cultivability inactivation. Optimal experimental conditions required a semiconductor concentration of 0.1 with 3.8 mW/cm(2) UV-A radiance intensity. Cell viability was monitored by plasma membrane permeability and loss of enzymatic activity using double fluorescent dye staining and flow cytometry. Plasma membrane permeability and enzymatic activity were almost simultaneously targeted. Esterase enzymatic activity decreased progressively, suggesting that the intracellular protein pool was sequentially damaged by the treatment. In yeast cells, the presence of a thick cell wall did not prevent the plasma membrane from being a prime photocatalytic target. Monitoring of chemical byproducts confirmed the loss of membrane integrity. A massive loss of potassium, major cation in yeast, that was released first and in large amounts, remained constant beyond 5 h of treatment and was correlated to the number of damaged membrane-cells. On the contrary, ammonium ions concentration gradually increased, suggesting their generation through the photocatalytic process, probably via amino acid and protein degradation. Oxamic and oxalic acid that could also arise from damages to amino acids were detected. Amino acid analysis revealed a main component, increasing over time, corresponding to glycine. Glycine could be produced via the transformation of other amino acids, released from cell wall, membrane and intracellular proteins. (C) 2013 Elsevier B.V. All rights reserved.