**2. Kombucha biological structure**

Kombucha microbial consortium (KMC), is a fermented tea and non-alcoholic beverage prepared with water, tea, sugar and kombucha culture (Symbiotic Consortium Of Bacteria and Yeasts known as SCOBY or tea fungus). The Kombucha community is not found in nature. It has been originated by mankind and known already around 220 B.C in Manchuria, northeast China, when it was appreciated for its detoxifying and energising properties [16]. Kombucha microbial consortium is an example of the advanced mutualistic interactions between representatives of two kingdoms of living organisms: bacteria and yeasts [17] (**Figure 1**) . The core bacterial community within kombucha brewing is dominated by bacteria *Komagataeibacter, Acetobacter,* and *Gluconobacter* [19]. It may contain lactic acid bacteria [20, 21], but the most remarkable genera characterised in kombucha microbial consortium are *Bacteroides* and *Prevotella* known as dominant human gut microbiota species [22]. Recent metagenomics screening predicted the presence of opportunistic bacteria like *Bacillus, Pseudomonas* etc. [23, 24], bacteriophages and even yeast viruses [25]. Yeast fraction changes in genera composition depending on geographical origin, and it consists of representatives of *Zygosaccharomyces, Brettanomyces, Schizosaccharomyces, Saccharomyces*, and *Pichia* [19]. The biofilm, as a three-dimensional microbial hydrogel settlement, supports an evolutionary stable social cooperation between its inhabitants, in a way that is analogous to tissues in multicellular organisms. The biofilm may also optimise oxygen concentrations for the microbial microcolonies, which are stratified in the cellulosic matrix, as well as

#### **Figure 1.**

*Kombucha microbial consortium consists of multiple symbiotic species of bacteria and yeasts. On the surface acetic bacteria synthesise the most abundant polymer on our planet - cellulose (modified from [18]).*

protect the community from environmental stressors, such as UV radiation [26]. The capability of kombucha microbial consortium to generate and tolerate acidic conditions, metabolise ethanol, and produce organic acids, protects the system from invasion by competitor microbes. The bacterially-produced cellulose-based pellicle biofilm may also provide protection from cheaters by inhibiting the diffusion of their extracellular metabolites.
