Which Bacteria Are Present in Kefir?

January 1, 2026 •

2 min read

Kefir grains contain a highly complex and symbiotic microbial community of bacteria and yeast that co-exist within a polysaccharide matrix. Understanding which bacteria are present in kefir reveals why this fermented drink is so beneficial for human health, offering a wider range of probiotic species than many commercial yogurts.

Quick Summary

Kefir contains a diverse array of beneficial bacteria, primarily lactic acid and acetic acid bacteria, alongside various yeasts. The specific microbial composition depends on the kefir grain's origin and the fermentation process, offering a wide range of probiotics for gut health.

Key Points

Rich Microbial Diversity: Kefir contains a highly diverse population of beneficial bacteria and yeasts, often exceeding the variety found in commercial yogurts.
Dominant Bacteria: Key bacterial groups include lactic acid bacteria (LAB) like Lactobacillus and Lactococcus species, as well as acetic acid bacteria (AAB).
Role of Symbiosis: The bacteria and yeasts in kefir grains exist in a symbiotic relationship, with each organism contributing to the fermentation process and the overall stability of the culture.
Milk vs. Water Kefir: Milk kefir generally has a wider variety of probiotics, while water kefir is a suitable dairy-free alternative with a different, though still beneficial, microbial profile.
Health Benefits: The bacteria in kefir contribute to its probiotic effects, which have been linked to improved gut health, antimicrobial activity, and immunomodulatory properties.

A Complex Ecosystem: The Kefir Grain Microbiota

Kefir's unique properties and potent probiotic benefits stem from the complex symbiotic culture of bacteria and yeast (SCOBY) held within its grains. These cauliflower-like grains provide a stable environment where a multitude of microorganisms thrive in a cooperative relationship. The microbial composition can vary significantly based on factors like the type of milk or sugar used, the grain's origin, and fermentation conditions.

Key Bacterial Species in Kefir

Most bacteria in kefir fall into lactic acid bacteria (LAB) and acetic acid bacteria (AAB) categories.

Lactic Acid Bacteria (LAB)

LAB are abundant and ferment lactose into lactic acid, giving kefir its tangy taste and acidity. Noteworthy species include Lactobacillus kefiranofaciens, which produces kefiran, the grain's structural polysaccharide, Lactobacillus kefiri, Lactobacillus plantarum, Lactococcus lactis, Leuconostoc mesenteroides, Streptococcus thermophilus, and Bifidobacterium species.

Acetic Acid Bacteria (AAB)

AAB are also present and produce acetic acid, contributing to the sour profile. Common AAB in milk kefir grains include Acetobacter aceti and Acetobacter rasens, while water kefir may contain species like Acetobacter fabarum.

Milk Kefir vs. Water Kefir: A Microbial Comparison

While both use SCOBYs, their microbial makeup differs based on the fermentation base.


Feature	Milk Kefir	Water Kefir
Base Medium	Animal milk (cow, goat, sheep)	Sugar water (brown sugar, molasses, juice)
Primary Microorganisms	Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, Bifidobacterium, and yeasts	Lactobacillus (including L. brevis, L. casei), Leuconostoc, Acetobacter, and yeasts
Primary Fermentation Products	Lactic acid, some ethanol, CO2, plus milk nutrients	Lactic acid, acetic acid, ethanol, and CO2, fewer nutrients than milk kefir
Probiotic Diversity	Generally greater diversity due to milk's nutrient profile	Robust, but typically fewer species than milk kefir
Health Benefits	Wider range of benefits, including improved lactose digestion	Hydration and a dairy-free option for those with intolerance or vegan diets

The Role of Symbiosis

In kefir grains, bacteria and yeast form a symbiotic relationship. LAB produce lactic acid, creating an acidic environment. AAB and yeasts, such as Saccharomyces cerevisiae, consume some lactic acid, raising the pH and allowing other bacteria to flourish. This balance is crucial for the grains' survival and the final product's health benefits.

The Health Benefits Linked to Kefir Bacteria

The bacteria in kefir are associated with various health benefits, including improved gut immunity, antimicrobial effects, and anti-inflammatory properties. Certain Lactobacillus strains from kefir can inhibit pathogens. Kefiran is also linked to positive health outcomes. The diverse and concentrated microorganisms in kefir are thought to modify the gut microbiota for a healthier digestive system.

Conclusion

Kefir's beneficial symbiotic community of LAB, AAB, and yeasts is the foundation of its health advantages. Milk kefir generally offers more diverse strains, while water kefir is a valuable dairy-free alternative with its own beneficial microorganisms. While specific species vary, the symbiotic ecosystem produces a potent probiotic food supporting gut and immune health. Kefir exemplifies food as functional medicine, directly tied to its microbial inhabitants. For further reading, research from Frontiers in Microbiology provides insights into kefir's microbial composition.

Frequently Asked Questions

The primary difference lies in the diversity and specific strains. Milk kefir generally has a broader range of probiotic strains, including specific Lactobacillus species adapted to a dairy environment. Water kefir contains a different, though still beneficial, set of bacteria and is an excellent dairy-free option.

Kefiran is a unique, water-soluble polysaccharide that forms the gelatinous matrix of kefir grains. It is primarily produced by the bacterium Lactobacillus kefiranofaciens, a key species within the kefir ecosystem.

Yes, various Bifidobacterium species, such as Bifidobacterium bifidum, have been identified in kefir, particularly milk kefir. They contribute to the probiotic richness of the final beverage.

Acetic acid bacteria (AAB), such as Acetobacter species, work alongside lactic acid bacteria and yeasts. They produce acetic acid, which contributes to the sharp flavor profile of kefir and is a key component of the symbiotic fermentation process.

They form a stable symbiotic relationship. Lactic acid bacteria create an acidic environment, which is then managed by acetic acid bacteria and yeasts that consume the acids and help maintain a balanced pH, allowing the entire community to thrive.

Traditional, grain-fermented kefir typically contains a greater diversity of bacterial and yeast strains compared to many commercial yogurts, which are often made with a smaller number of selected starter cultures.

The bacterial composition can be influenced by several factors, including the grain's origin, the type of milk or sugar used, the fermentation temperature, and the duration of fermentation.