Saccharomyces cerevisiae does not undergo a quorum sensing-dependent switch of budding pattern.

Saccharomyces cerevisiae can alter its morphology to a filamentous form associated with unipolar budding in response to environmental stressors. Induction of filamentous growth is suggested under nitrogen deficiency in response to alcoholic signalling molecules through quorum sensing. To investigate this further, we analysed the budding pattern of S. cerevisiae cells over time under low nitrogen conditions while concurrently measuring cell density and extracellular metabolite concentration. We found that the proportion of cells displaying unipolar budding increased between local cell densities of 4.8 × 106 and 5.3 × 107 cells/ml. This increase in unipolar budding was not reproduced with cells growing at the critical cell density and in conditioned media. Growth under high nitrogen conditions also resulted in increased unipolar budding between local cell densities of 5.2 × 106 and 8.2 × 107 cells/ml, but with differences in metabolite concentration compared to low nitrogen conditions. Neither cell density, metabolite concentration, nor nitrogen deficiency were therefore sufficient to increase unipolar budding. Therefore, by using the budding pattern as an early indicator of filamentous growth, our results suggest that quorum sensing may not control the switch of budding behaviour in S. cerevisiae. Only a high concentration of the putative signalling molecule, 2-phenylethanol, resulted in an increase in unipolar budding. However, this concentration was not physiologically relevant, suggesting toxicity rather than a known quorum sensing mechanism.

Reliable budding pattern classification of yeast cells with time-resolved measurement of metabolite production.

Filamentous growth in Saccharomyces cerevisiae is a stress response commonly induced under nutrient deprivation and by certain alcohols. It is a compound phenotype characterized by pseudohyphal growth, invasion and a shift to more polarized budding. Previous methods have not allowed the time-resolved determination of filamentous growth. Here we present a new method for budding pattern characterization that enables the measurement of filamentous growth and metabolite concentration during yeast cell growth at precise time intervals. By combining chemical cell immobilization and single-cell imaging using an oCelloScope™, this method provides more accurate budding pattern classification compared with previous methods. The applications of the method include, for example, investigation of quorum sensing-controlled yeast filamentous growth and metabolism under stress and identification of toxic metabolites.

Can community-based signalling behaviour in Saccharomyces cerevisiae be called quorum sensing? A critical review of the literature.

Quorum sensing is a well-described mechanism of intercellular signalling among bacteria, which involves cell-density-dependent chemical signal molecules. The concentration of these quorum-sensing molecules increases in proportion to cell density until a threshold value is exceeded, which triggers a community-wide response. In this review, we propose that intercellular signalling mechanisms can be associated with a corresponding ecological interaction type based on similarities between how the interaction affects the signal receiver and producer. Thus, we do not confine quorum sensing, a specific form of intercellular signalling, to only cooperative behaviours. Instead, we define it as cell-density-dependent responses that occur at a critical concentration of signal molecules and through a specific signalling pathway. For fungal species, the medically important yeast Candida albicans has a well-described quorum sensing system, while this system is not well described in Saccharomyces cerevisiae, which is involved in food and beverage fermentations. The more precise definition for quorum sensing proposed in this review is based on the studies suggesting that S. cerevisiae may undergo intercellular signalling through quorum sensing. Through this lens, we conclude that there is a lack of evidence to support a specific signalling mechanism and a critical signal concentration of these behaviours in S. cerevisiae, and, thus, these features require further investigation.