Water Kefir as a Model for Aggregative Development of Microbial Multicellularity within a Multispecies Biofilm

Water kefir, a fermented beverage and dairy-free alternative to milk kefir, consists of cauliflower-like aggregates known as water kefir grains. These grains, measuring between 1-20 mm, are composed of an exopolysaccharide matrix formed from the fermentation substrate sucrose. The microbial community dwelling on and within these grains forms a complex, highly stable multicellular aggregate, predominantly comprising 10-15 core species.

These species belong to three primary groups: lactic acid bacteria, acetic acid bacteria, and yeasts. The unique organizational structure, stability, and growth of kefir grains cannot be simply attributed to the individual species; attempts to recreate the grains solely from isolated species have proven unsuccessful. To shed light on this intriguing phenomenon, researchers have now combined a detailed examination of the macro- and micromorphology of water kefir grains with a variety of analyses of its microbial biodiversity.

By employing techniques such as flow cytometry, fluorescence-activated cell sorting, and amplicon sequencing of selected subpopulations, scientists have been able to study the dynamics and interactions within this intricate microbial community. Moreover, they have quantitatively characterized the growth of water kefir grains over time and space, revealing an emergent multicellular growth function that has been documented for the first time.

This comprehensive understanding of the highly structured and metabolically synergistic microbial aggregate offers valuable insights into the development of microbial multicellularity from multiple species. The findings provide a solid foundation for future mechanistic studies aimed at unraveling the self-organizing processes that lead to the formation of stable, spatially structured multicellular aggregates within multispecies microbial communities.