Impact of cooperative or competitive dynamics between the yeast Saccharomyces cerevisiae and lactobacilli on the immune response of the host

Abstract

Fungi and bacteria can be found coexisting in a wide variety of environments. The combination of their physical and molecular interactions can result in a broad range of outcomes for each partner, from competition to cooperative relationships. Most of these interactions can also be found in the human gastrointestinal tract. The gut microbiota is essential for humans, helping the assimilation of food components as well as the prevention of pathogen invasions through host immune system modulation and the production of beneficial metabolites such as short-chain fatty acids (SCFAs). Several factors, including changes in diet habits due to the progressive Westernization of the lifestyle, are linked to the onset of dysbiosis statuses that impair the correct balance of the gut environment. It is therefore crucial to explore the interactions between commensal and diet-derived microorganisms and their influence on host health. Investigating these interactions through co-cultures between human- and fermented food-derived lactobacilli and yeasts led us to understand how the strains' growth yield and their metabolic products rely on the nature and concentration of the species involved, producing either cooperative or competitive dynamics. Moreover, single cultures of yeasts and lactobacilli proved to be ideal candidates for developing immune-enhancing products, given their ability to induce trained immunity in blood-derived human monocytes in vitro. Conversely, co-cultures as well as mixtures of yeasts and lactobacilli have been shown to induce an anti-inflammatory response on the same immune cells in terms of cytokine profiles and activation surface markers, opening new possibilities in the design of probiotic and dietary therapies.

Keywords: Saccharomyces cerevisiae; fermented food; host immune system modulation; lactobacilli; microbial ecology; short-chain fatty acids; trained immunity; yeasts.

Figure 1

Counts over time of L. plantarum B1 in single culture and co-culture with S. cerevisiae YH1 in MRS medium at 37°C for 24 hours in triplicates, with a yeast-bacteria concentration ratio of 1:10 (A) or 1:1 (B). Cellular concentrations were determined by cell counting at the Bürker chamber. Statistical significance was assessed by T-test, * for p < 0.05.

Figure 2

Counts over time of L. delbrueckii TJA9 in single culture and co-culture with S. cerevisiae strains YH1 (A) and CL4 (B) in MRS medium at 37°C for 24 hours in triplicates. Cellular concentrations were determined by cell counting in the Bürker chamber. Statistical significance was assessed by T-test, * for p < 0.05.

Figure 3

Counts over time of S. cerevisiae strains YH1 (A) and CL4 (B) in single culture and co-culture with L. delbrueckii TJA9 in MRS medium at 37°C for 24 hours in triplicates, with a yeast-bacteria concentration ratio of 1:10. Cellular concentrations were determined by cell counting at the Bürker chamber. Statistical significance was assessed by T-test, * for p < 0.05.

Figure 4

SCFAs (Butyric acid, 2-MethylButyric acid, and Valeric acid) production in chemically defined medium represented as time-scaled points and relative linear regression models (dotted lines). The SCFAs production is plotted as concentration (on the y-axis) in 8 different time points (on the x-axis) for the three studied conditions. Blue stands for yeast single cultures, red stands for bacterial single cultures, and green stands for co-cultures. Each value is the mean of three different biological replicates. Statistical significance for each experiment is shown in Supplementary material ( Supplementary Tables S3 , S6 , S7 , S10 , S12 , S13 ).

Figure 5

TNF-ɑ production by healthy human monocytes after stimulation with diverse single, co-cultures and mixtures of the selected yeast and lactobacillus strains and subsequent stimulation with LPS after 5 days, compared to stimulation with LPS only (LPS columns). Graphs show means and standard errors for 15 independent experiments (N = 15). Statistical significance was assessed by one-way ANOVA; * for p < 0.05.

Figure 6

IL-6 production by healthy human monocytes after stimulation with diverse single, co-cultures and mixtures of the selected yeast and lactobacillus strains and subsequent stimulation with LPS after 5 days, compared to stimulation with LPS only (LPS columns). Cytokine production is expressed as fold increase compared to control. Graphs show means and standard errors for 15 independent experiments (N = 15). Statistical significance was assessed by one-way ANOVA; * for p < 0.05, ** for p < 0.01, *** for p < 0.001, **** for p < 0.0001.

Figure 7

TNF-ɑ and IL-10 productions by human monocytes at Day 6 after 24-hour incubation with single culture (A), co-cultures, and mixtures (B) of selected yeast and bacterial strains and a subsequent stimulus with LPS at Day 5. Graphs show means and standard errors for 6 independent experiments (N = 6). Statistical significance was assessed by the Mann-Whitney nonparametric test; ** for p < 0.01.

Figure 8

Surface markers pattern of human monocytes at Day 6 after 24-hour incubation with single culture, co-cultures, and mixtures of selected yeast and bacterial strains and a subsequent stimulus with LPS at Day 5. Results are expressed as the percentage of cells that showed a specific marker on their surface. Percentages of positive cells are reported on a color scale from the lower values (blue) to the higher ones (red).