What strains of bacteria does kefir have?

December 28, 2025 •

4 min read

Kefir grains contain a highly complex and symbiotic community of microbes, with one study identifying as many as 61 different species of bacteria and yeast. This incredible microbial diversity is what makes kefir a potent probiotic, offering a wealth of beneficial bacteria that supports digestive wellness and overall health.

Quick Summary

Kefir contains a diverse array of beneficial bacteria, including numerous species from the genera Lactobacillus, Lactococcus, Streptococcus, and Bifidobacterium, which contribute to its probiotic properties. The specific strains can vary based on factors like the type of kefir (milk or water), the kefir grains' origin, and the fermentation process. These microorganisms work synergistically to provide digestive and immune system support.

Key Points

Lactobacilli are Dominant: The most prominent bacterial group in kefir is Lactobacillus, which includes species like L. kefiranofaciens, L. kefiri, and L. acidophilus.
Kefiran Production: Lactobacillus kefiranofaciens is the key strain responsible for producing kefiran, the polysaccharide that forms the gelatinous structure of kefir grains.
A Symbiotic Community: Kefir's bacteria coexist in a symbiotic relationship with yeasts and other microbes within the kefir grain, creating a more potent probiotic effect than isolated strains.
Variety Based on Fermentation: The specific bacterial strains present can vary depending on the fermentation base. Milk kefir contains dairy-specific bacteria like Lactococcus lactis, while water kefir includes strains like Acetobacter.
Diverse Health Benefits: The wide array of bacteria in kefir contributes to its various health benefits, including improved digestion, enhanced immunity, and antimicrobial properties.
Supports Gut Health: By introducing a diverse microbial population, the bacteria in kefir help promote a healthy, balanced gut microbiome.

Kefir is a fermented beverage renowned for its complex and diverse microbiota, which is primarily responsible for its health-promoting properties. The specific microbial composition of kefir can vary greatly depending on its origin, the type of milk or sugar solution used, and fermentation conditions. A deeper dive reveals that these probiotic-rich beverages are teeming with a wide range of bacteria, mainly from the lactic acid bacteria (LAB) family, alongside yeasts and some acetic acid bacteria.

The Primary Bacterial Genera in Kefir

Lactobacillus Species

Lactobacilli are the most abundant and well-studied bacterial group in kefir. They are crucial for breaking down sugars, such as lactose, into lactic acid, which gives kefir its characteristic tangy taste. Some of the most frequently identified Lactobacillus strains include:

Lactobacillus kefiranofaciens: This species is a key producer of kefiran, a unique polysaccharide that forms the gelatinous matrix of kefir grains and possesses its own bioactive properties.
Lactobacillus kefiri: Often a dominant species, L. kefiri contributes significantly to the fermentation process and has been shown to exhibit antibacterial effects against pathogens like Listeria monocytogenes.
Lactobacillus acidophilus: Known for its ability to survive the harsh conditions of the gastrointestinal tract, L. acidophilus is a well-regarded probiotic that can aid in nutrient absorption.
Lactobacillus helveticus: This strain is frequently found in kefir and is recognized for its potential to produce bioactive peptides with various health benefits.
Lacticaseibacillus paracasei (formerly Lactobacillus paracasei): Present in both milk and water kefir grains, this species is noted for its antimicrobial properties.

Lactococcus and Streptococcus Species

In addition to Lactobacilli, other LAB play vital roles. Species from the genera Lactococcus and Streptococcus are often present, especially in milk kefir.

Lactococcus lactis: A major component, L. lactis is one of the most abundant bacterial species in fermented kefir milk, and contributes to the initial phases of fermentation.
Streptococcus thermophilus: Commonly found in dairy products, S. thermophilus also contributes to the fermentation process.

Bifidobacterium

Some kefir products and grains also contain Bifidobacterium species. Though not as prominent in traditional preparations, they can be added through specific starter cultures to enhance the probiotic profile. Bifidobacterium species are known to promote a healthy balance in the gut microbiome. Common species include Bifidobacterium bifidum and Bifidobacterium longum.

Bacteria in Water Kefir vs. Milk Kefir

While the general microbial structure is similar, the precise bacterial strains present differ between milk and water kefir, primarily because of the different fermentation substrates. Water kefir uses a sugar-water solution, while milk kefir uses lactose from milk as its food source.

Milk Kefir's Unique Strains: Lactobacillus kefiranofaciens is a dominant species in milk kefir grains, producing kefiran from milk's lactose. The higher protein and fat content of milk also supports a different bacterial community.

Water Kefir's Unique Strains: Water kefir grains contain a different set of bacteria, including species of Lactobacillus, Leuconostoc, and Acetobacter. Acetobacter species, for example, are more common in water kefir due to its aerobic fermentation stage and contribute to the acetic acid content.

Comparison Table: Milk Kefir vs. Water Kefir Bacteria


Feature	Milk Kefir	Water Kefir
Dominant LAB	Lactobacillus kefiranofaciens, L. kefiri, Lactococcus lactis	Lactobacillus brevis, L. hordei, L. nagelii
Other Bacteria	Bifidobacterium spp., Streptococcus spp.	Leuconostoc spp., Acetobacter spp.
Primary Food Source	Lactose (milk sugar)	Sucrose, fructose, glucose (from sugar-water solution)
Resulting Flavor Profile	Creamy, tangy, slightly yeasty	Light, bubbly, slightly acidic, less creamy

The Symbiotic Relationship in Kefir Grains

Kefir's bacterial diversity is sustained by the symbiotic relationship within the kefir grain itself, which is a complex matrix of polysaccharides (kefiran), lipids, and proteins. The bacteria and yeast co-exist and thrive by sharing metabolic byproducts. For instance, yeast activity can provide growth factors that benefit the lactic acid bacteria. This synergistic interaction is more stable and biologically active than a simple mixture of the individual microorganisms, contributing to kefir's overall probiotic power.

Conclusion: The Dynamic and Diverse World of Kefir Bacteria

In conclusion, the answer to "what strains of bacteria does kefir have?" is complex, highlighting a wide and dynamic range of microbial species. From the ubiquitous Lactobacillus species like kefiranofaciens and kefiri to co-existing Lactococcus, Streptococcus, and Bifidobacterium strains, kefir is a microbially-rich food. These probiotic bacteria, supported by a symbiotic community of yeasts, provide diverse health benefits. While the precise composition varies with fermentation, the foundational blend of bacteria consistently offers a powerful way to support gut health and overall well-being. The dynamic nature of kefir's microbiome makes it a particularly fascinating and effective fermented food.

For further reading on the microbial composition and health benefits of kefir, consult the review article published in Frontiers in Microbiology.

Frequently Asked Questions

No, the bacterial composition of kefir is highly variable. Factors such as the origin of the kefir grains, the fermentation substrate (milk versus water), and environmental conditions all influence the specific strains and proportions of bacteria present.

The primary group of bacteria found in kefir is the lactic acid bacteria (LAB), with various species of the Lactobacillus genus being the most predominant. These are responsible for the fermentation of lactose into lactic acid.

Yes, the bacteria differ based on the fermentation medium. Milk kefir grains contain bacteria suited for lactose, like Lactobacillus kefiranofaciens, while water kefir grains host bacteria that thrive on sugar water, such as certain Lactobacillus and Acetobacter species.

Pasteurized commercial kefirs often use specific starter cultures containing a more standardized and limited range of bacteria compared to the highly diverse, wild strains found in traditional, homemade kefir grains.

Kefiran is a unique polysaccharide that forms the gelatinous matrix of kefir grains. It is primarily produced by the bacterium Lactobacillus kefiranofaciens and contributes to kefir's thick texture and health benefits.

Yes, some kefir products, particularly commercial varieties, are supplemented with Bifidobacterium species. These strains contribute to the probiotic profile and support gut health.

The high bacterial diversity in kefir provides a wide array of probiotics that promote a balanced gut microbiome, aid digestion, boost immune function, and offer antimicrobial properties.