Short answer
Kombucha is fermented by a mix of acetic acid bacteria, lactic acid bacteria and yeasts. The main acetic acid bacterium is Komagataeibacter xylinus, formerly called Gluconacetobacter. It turns sugar into acids and also builds the cellulose mat that forms the SCOBY. The yeasts make carbon dioxide and a little alcohol.
The acetic acid bacteria
Acetic acid bacteria do most of the acid work. Komagataeibacter xylinus produces acetic acid and spins the cellulose that forms the SCOBY, the floating disc that houses the culture. Other acetic acid bacteria from the same group also contribute. The acid lowers the pH of the brew, which helps keep it safe from most spoilage organisms.
The lactic acid bacteria
Lactobacillus species are found in many kombucha cultures. They add to the acidity and to the flavour. Lactic acid bacteria are also common in yoghurt and sauerkraut, so the names will sound familiar.
The yeasts
The yeasts first break sucrose into glucose and fructose. They then turn the sugar into ethanol and carbon dioxide. The carbon dioxide is what makes the fizz in the second ferment. Research on kombucha has named several yeast species, including Zygosaccharomyces, Brettanomyces (also called Dekkera), Pichia and Saccharomyces.
How the mix works together
The yeasts make a little alcohol and carbon dioxide from the sugar. The bacteria then turn that alcohol into acids. As the acids build up, the drink becomes more sour and there is less sugar left. This balance is why a kombucha that ferments for longer tastes more tart.
Does every culture have the same mix?
No. The mix varies with the SCOBY, the tea, the sugar and the region where the culture was kept. Two SCOBYs can give different flavours and different fizz. That is one reason many brewers keep and share starter tea from a batch they like.
Do the bacteria survive your stomach?
Probably not in large numbers. A gastroenterologist at UW Medicine has noted that most kombucha bacteria are unlikely to survive stomach acid. Research on kombucha as a probiotic is limited, so the drink should not be treated as a substitute for a probiotic supplement. It can still be a pleasant, fermented drink, and that is a fair reason to enjoy it.
Where to get a healthy culture
Ask a friend who brews, join a local brewing group, or buy a SCOBY from a reputable supplier. What a kombucha starter is explains what you should receive. Avoid SCOBYs that have been dried or refrigerated, unless the seller says they are viable. A healthy SCOBY should have come from a clean, active batch.
Where to go next
For a simple definition of the drink, see what is kombucha. For how to store a SCOBY between batches, see how long a SCOBY can sit in starter liquid. For the full method, see how to flavour kombucha.