Sourdough starters are spontaneously fermented mixtures of flour and water. In the stable communities that emerge from this fermentation, the yeast Maudiozyma humilis (formerly Kazachstania humilis) and lactic acid bacterium Fructilactobacillus sanfranciscensis co-occur frequently. Because the former cannot metabolise maltose (the most common small carbohydrate in dough) the cross-feeding of glucose to the yeast has been presumed to facilitate a trophic relationship between the two.
We examined the metabolomics and growth kinetics of M. humilis and F. sanfranciscensis co-cultures in a synthetic sourdough medium (SSM). Combinations of 8 strains of each species (plus mono-cultures) were grown in SSM for 24 h, then cells were stained and counted via flow cytometry. Carbohydrates, organic acids, and pH of the spent medium were quantified. One yeast-bacteria pair was studied further. Growth curves were created at 25 or 30°C in media adjusted to a pH of 3, 4, 5, or 6 with lactic acid. Then, cell-free conditioned media was prepared by growing one microbe in SSM, removing the cells, then growing the other microbe in it.
Flow cytometry data reveal that yeast viability was greater in mono-culture than when grown with F. sanfranciscensis. Concentrations of maltose, glucose, and lactate showed that competition for nutrients in a limited ecosystem was potentially associated with greater yeast mortality. Furthermore, bacterial strain identity tended to determine ratios of live and dead microbes in all co-culture pairings. Temperature also had the greatest effect on yeast and bacterial growth kinetics, regardless of whether the SSM was pre-conditioned.
Therefore, in a nutrient-limited context, the interaction between M. humilis and F. sanfranciscensis is not ecologically positive. In a real dough context, which removes the need for competition for resources, temperature and pH optima are likely to affect each microbe individually.