Résumé
During microbial cell cultures, environmental conditions affect cell physiology and subsequently process efficiency. Physiological changes result in changing cell morphology, such as cell size variations. The aim of this work was to study cell size evolution of a Saccharomyces cerevisiae population exposed to various stresses during alcoholic batch fermentations, and to evaluate the potential use of cell size measurements to infer cell viability. During a reference culture, without perturbation, viability as assessed by propidium iodide staining (PI) remained 100% and mean cell diameter was found to be above 5µm. A rapid temperature shift from 33 to 43 ◦C at 50gl−1 of ethanol resulted in an immediate arrest of growth and triggered a progressive loss of viability from 100% to 0% and a decrease of mean cell diameter from 5.2 to 3.7µm. Cell size distribution curves obtained with a cell counter showed an increasing subpopulation of significantly smaller cells. At single-cell level, combined microscopy size measurements and PI staining showed that this subpopulation was exclusively composed of dead cells. Similar results were obtained after acetic acid or furfural additions. Accordingly, a multivariate data analysis was achieved to estimate the ratio of dead cells from cell size distributions obtained using the cell counter.