When you take a refreshing and effervescent sip of kombucha, you’re actually ingesting the result of a complex model ecosystem of microbes, including both bacteria and fungi. Model ecosystems help scientists and society reveal the mechanisms and interactions behind the complexity of the natural world.
The first known records of kombucha are found in early 19th century Russia, though the origin is still uncertain. To make kombucha, sugar is added to black or green tea. After it cools, the tea is supplemented with liquid from a previous batch of kombucha (about 10% of the volume of the freshly brewed tea). A “mother”, or SCOBY (Symbiotic Community of Bacteria and Yeast) is placed on top of the liquid and then allowed to ferment for about two weeks.
A new review published earlier this month by scientists from Arizona State University, University of New Mexico and University of Cape Town, South Africa summarizes why kombucha makes an excellent microbial model ecosystem. Defined by both cooperation and conflict, here’s why kombucha is a new model system for microbial ecosystems:
First, yeast produce an enzyme that is a “public good”.
The enzyme, invertase turns sucrose into fructose and glucose. This reaction is the first metabolic step of many that benefits the microbial community. Invertase is released into the liquid of the kombucha, making it a public good, or something that benefits both the yeast and bacteria in the kombucha.
Since it is a public good, there can be conflict: some “cheater” yeast don’t actually produce the invertase. However, cheater yeast perform worse in cultures with bacteria. Hard-working yeast producing invertase do benefit from their investment in themselves and bacteria.
Next, bacteria turns those sugars into a cellulose that forms a film on the surface of the kombucha.
This essentially stores the energy for later use, again, by both the yeast and bacteria. The biofilm that forms on top of the kombucha also provides protection from other microbes which could invade. Moreover, the biofilm increases access to oxygen for the microbes that are living within the biofilm itself.
Finally, the alcohol and acids generated by fermentation provides even more protection against contamination by other microbes.
Yeast make alcohol (thank them for beer, wine and other alcohols) while bacteria make acidic compounds (thank them for delicious sour beers). This contributes to a relatively unfavorable environment for other microbes that happen to land in the culture. The bacterial and fungal species in the SCOBY work together to prevent other microbes from taking over the sugary liquid which is good for growth.
Interestingly, the antimicrobial properties do not degrade even when the culture is adjusted to a neutral pH and heated to 80 degrees Celsius for half an hour.
Existing microbial model ecosystems usually only use a single species, or just a few. Kombucha cultures can contain many more species that interact in a complex, but reproducible way.
The authors point out that since kombucha is easily shared, non-toxic and inexpensive, it is an excellent model ecosystem for citizen scientists.