The culture mainly contains a symbiosis of Acetobacter (acetic acid bacteria) and one or more yeasts.
The culture itself looks somewhat like a large pancake, and though often called a mushroom, a mother of vinegar or by the acronym SCOBY (for "Symbiotic Colony of Bacteria and Yeast"), it is scientifically classified as a zoogleal mat. It takes on the shape of its container, but varies in thickness depending on how long it has been allowed to develop and the acidity of the tea medium during the development period. The culture is leathery and non-elastic, similar to a thick calamari.
The yeast component of kombucha may contain any of Saccharomyces cerevisiae, Brettanomyces bruxellensis, Candida stellata, Schizosaccharomyces pombe, Torulaspora delbrueckii, and Zygosaccharomyces bailii, or another domesticated strain. Alcohol production by the yeast(s) contributes to the production of acetic acid by the bacteria. Alcohol concentration also plays a role in triggering cellulose production by the bacterial symbionts.
The bacterial component of a kombucha culture usually consists of several species, but will almost always contain Gluconacetobacter xylinus (formerly Acetobacter xylinum), which ferments the alcohols produced by the yeast(s) into acetic acid. This increases the acidity while limiting the alcoholic content of kombucha. G. xylinum is responsible for most or all of the physical structure of a kombucha mother, and has been shown to produce microbial cellulose. This is likely due to artificial selection by brewers over time, selecting for firmer and more robust cultures.
The acidity and mild alcoholic element of kombucha resists contamination by most airborne molds or bacterial spores. As a result, kombucha is relatively easy to maintain as a culture outside of sterile conditions. The bacteria and yeasts in kombucha may also produce antimicrobial defense molecules. Gluconacetobacter diazotrophicus, a bacterium related to G. xylinum, is known to produce an antimicrobial known as a bacteriocin.
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