#FEMSmicroBlog: What's in your kombucha?


In recent years, kombucha gained a lot of popularity in the Western world. Originating in the Orient, this fermented tea is a traditional non-alcoholic drink with many claimed health benefits. Since many microbe-lovers started to make their own kombucha at home, Sarah Wettstadt explores for the #FEMSmicroBlog what kombucha actually is and which microbes help make this popular drink. #FascinatingMicrobes


What’s the deal with kombucha

In recent years, kombucha tea gained more shelf space in supermarkets and many guides now explain in detail how to make kombucha at home. One can easily prepare kombucha from green or black tea with added sugar. The static fermentation of kombucha usually takes 7 – 15 days at room temperature. After that, the final product has a slightly acidic or sour taste.

Kombucha tea was shown to have beneficial effects like anti-microbial, anti-oxidant, anti-carcinogenic, anti-diabetic in animal and cell models. However, direct health benefits from controlled studies in humans are currently lacking.

Beneficial effects of kombucha were shown in animal and cell models while studies in humans are still lacking.

The microbes needed for the fermentation process swim in the kombucha broth and reside in a cellulosic pellicle on top of the tea. With each fermentation process, the cellulosic pellicle gets a new layer of biofilm. This plus some broth from a previous batch are to be used as starter cultures in subsequent fermentations.


The microbiome of kombucha

The microbial community of kombucha tea lives in two niches: one community comprising the biofilm within the cellulosic pellicle and the second one residing in the broth. The cellulosic biofilm is located at the liquid-air interface and contains a mixed community known as the symbiotic culture of bacteria and yeast or SCOBY.

microscopy of kombucha microbiome
Microscopy image taken with a phone camera of a kombucha batch by Justine Dees.

The kombucha microbiome includes genera from acetic acid bacteria, yeast and lactic acid bacteria. Studies identified different microbial species depending on the kombucha sources, the tea used for fermentation, fermentation duration and sequencing techniques.

Using high throughput sequencing, Chakravorty et al. found that the genus Candida showed the highest prevalence independent of fermentation time and both in the biofilm and liquid. In comparison, Villarreal-Soto et al. identified fungal species mainly in the liquid phases with Brettanomyces bruxellensis, Zygosaccharomyces bailii and Schizosaccharomyces pombe being the dominant species.

Other microbes identified in the kombucha tea belong to the family Acetobacteraceae, with the dominant genera comprising Komagataeibacter, Gluconacetobacter and Gluconobacter. One study additionally identified bacteria from the Oscillatoriaceae, Bifidobacteriaceae and Ruminococcaceae families. The same study found Proteobacteria to be the dominant phylum in the soup, with Firmicutes, Cyanobacteria and Actinobacteria also being present. Other bacteria present in the soup belonged to the Bifidobacterium, Collinsella, Enterobacter, Weissella and Lactobacillus genera.


How microbes make kombucha

As with many other natural fermentation processes, producing kombucha starts with yeasts hydrolysing sucrose into glucose and fructose. From these, they produce ethanol and CO2, so that after seven days of fermentation, ethanol concentration reaches a maximum. But at around 0,.5%, it remains way lower than your average beer.

In the next phase, acetic acid bacteria and lactic acid bacteria become metabolically active. These convert glucose and fructose into organic acids like gluconic and acetic acids. Plus, they produce acetic acid and lactic acid from ethanol.

This leads to a drop of pH of the kombucha soup down to 1.88 at day 21 of fermentation. Such a low pH likely protects against contaminants since not many microbes can survive such conditions.

The low pH of kombucha protects the tea against contaminants since not many microbes can survive in these conditions.

At this stage, microbes in the kombucha soup produce secondary metabolites like flavonoids and the polyphenols theaflavin and thearubigin. As reducing compounds, theaflavin and thearubigin donate electrons to other compounds thus increasing the potential anti-oxidant activity of kombucha. Also, the conversion of thearubigin to theaflavin likely brings the colour change of kombucha tea from reddish-brown to light brown.


Kombucha as a microbial superproduct?

Interestingly, studies agree that the specific kombucha microbiome and its cross-feeding behaviour directly relates to the production of important secondary metabolites. For example, the genome of Komagataeibacter rhaeticus contains pathways for the biosynthesis of vitamins B1, B7 and B12 and Zygosaccharomyces bailii requires the B‐group vitamins for its growth. Hence, it was suggested that the vitamin biosynthesis by Komagataeibacter species allows for Z. bailii to grow, rendering this interaction an important cross-feeding pathway in kombucha.

So far, not much research has looked into the actual health benefits of kombucha. Nevertheless, it gains popularity amongst home brewers while methods for industry-scale kombucha fermentation are being established. And a yeast first described from a kombucha brew got its own name from the tea: Zygosaccharomyces kombuchaensis. So, let us cheer to microbes for providing us with such a refreshing drink!


About the author of this blog

Dr Sarah Wettstadt is a microbiologist-turned science writer and communicator working on various outreach projects and helping researchers talk and write about their scientific results. Her overall vision is to empower through learning: she shares scientific knowledge with both scientists and non-scientists and coaches scientists in writing about their research. Sarah is blog commissioner for the FEMSmicroBlog and was a social media editor for FEMS for 1.5 years. Previous to her science communication career, she worked as a postdoc in Marían Llamas’ lab on Pseudomonas aeruginosa’s ability to use heterologous iron sources and completed her PhD with Alain Filloux investigating the type 6 secretion system in Pseudomonas aeruginosa.

About this blog section

The section #FascinatingMicrobes for the #FEMSmicroBlog explains the science behind a paper and highlights the significance and broader context of a recent finding. One of the main goals is to share the fascinating spectrum of microbes across all fields of microbiology.

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