Arneborg, N. | Search bioRxiv

bioRxiv2023

Ex-Lactobacillus Strains with Intrinsic Propensity to Stabilize Pickering Oil-in-Water Emulsions

Knowledge of surface characteristics is a major step in the evaluation of bacterial cells for potential use as Pickering emulsion stabilizers. Here, the cell surface characteristics of 31 strains of the ex-Lactobacillus genus were studied with the aim of evaluating their intrinsic abilities to serve as Pickering stabilizers of oil-in-water emulsions. About 77.42% of the tested strains demonstrated relatively highly negative zeta potential (-43.76 mV [≤] zeta potential [≤] -19.23 mV), while [~]58% of the strains demonstrated high cell surface hydrophobicity (microbial adhesion to hexadecane or MATH [≥] 30%). By combining these findings, four different cell surface features were defined (I, II, II and IV). Strains mainly demonstrated the type I surface feature ([~]45%), with most expressing strongly negative zeta potential and high surface hydrophobicity (zeta potential < -15 mV and MATH [≥] 30%, respectively). It appeared that the abundance of negative charge on the surfaces of ex-Lactobacillus cells positively influences surface hydrophobicity. Assessment of intrinsic Pickering stabilization potential using 12 selected strains indicated that four strains showed profound droplet size stability. At least one strain was observed to have natural propensity to form relativley compact and small emulsion droplets (63{+/-}3 {micro}m), leading to enhanced firmness and storage stability of the Pickering emulsions.

microbiology↗

bioRxiv2021

Clarifying intercellular signalling in yeast: Saccharomyces cerevisiae does not undergo a quorum sensing-dependent switch to filamentous growth

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 a quorum sensing mechanism. To investigate this claim, we analysed the budding pattern of S. cerevisiae cells over time under low nitrogen 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 4.8x106 and 5.3x107 cells/ml. However, the observed increase in unipolar budding could not be reproduced when cells were prepared at the critical cell density and in conditioned media. Removing the nutrient restriction by growth under high nitrogen conditions also resulted in an increase in unipolar budding between local cell densities of 5.2x106 and 8.2x107 cells/ml, but there were differences in metabolite concentration compared to the low nitrogen conditions. This suggests that neither cell density, metabolite concentration, nor nitrogen deficiency were necessary or sufficient to increase the proportion of unipolar budding cells. It is therefore unlikely that quorum sensing is the mechanism controlling the switch to filamentous growth in S. cerevisiae. Only a high concentration of the putative signalling molecule, 2-phenylethanol resulted in an increase in unipolar budding, but this concentration was not physiologically relevant. We suggest that the compound 2-phenylethanol acts through a toxicity mechanism, rather than quorum sensing, to induce filamentous growth. IMPORTANCEInvestigating dimorphism in the model organism Saccharomyces cerevisiae has been instrumental in understanding the signalling pathways that control hyphal growth and virulence in human pathogenic fungi. Quorum sensing was proposed to signal morphogenesis in S. cerevisiae populations. This mechanism requires the switch to filamentous growth to occur at a critical quorum sensing molecule concentration corresponding to a critical cell density. However, evidence for this mechanism is sparse and limited by the use of non-physiologically relevant concentrations of signalling metabolites. Our study designed a methodology to address this gap and may be applied to further studies of dimorphism in other types of yeasts. A significant implication of our findings is that morphogenesis does not occur in S. cerevisiae via a quorum sensing mechanism, and this important definition needs to be corrected. Mechanistic studies to understand dimorphism in yeasts, by considering metabolite concentrations, will further shed light onto this important cellular behaviour.

microbiology↗