Water kefir is a mildly sour and alcoholic drink fermented by a stable microbial multispecies community.
With its high sugar content and low amino acid concentration water kefir medium represents a
demanding habitat. In this ecological niche only well adapted microorganisms which are fit to the
consortium are able to grow and mutually provide essential nutrients. The synergism between main
representatives of water kefir yeasts and lactobacilli was studied in a co-culture model system. Cocultivation
of yeasts and lactobacilli in water kefir medium significantly increased cell yield of all
interaction partners, delineating the interaction of these water kefir isolates as mutualism. The support of
Zygotorulaspora (Z.) florentina was due to the acidification of the medium by the lactobacilli, whereas
lactobacilli are improved in growth by the disposal of essential nutrients produced by yeasts. The trophic
interaction between Lactobacillus (Lb.) hordei and yeasts is constituted by the release of amino acids and
Vitamin B6 from yeasts, whereas Lb. nagelii is supported in growth by their production of amino acids.
The interaction of Z. florentina and Lb. nagelii was further examined to reveal that co-cultivation induced
the yeast to release arginine, which was essential for Lb. nagelii.