What bacteria does Bulgarian yogurt have? Unpacking the probiotics

December 4, 2025 •

4 min read

For millennia, humans have consumed microorganisms through fermented foods like Bulgarian yogurt. The iconic taste and health benefits of this specific cultured dairy product come from a symbiotic relationship between two key bacterial strains: Lactobacillus bulgaricus and Streptococcus thermophilus.

Quick Summary

Bulgarian yogurt is defined by its two unique starter cultures: Lactobacillus bulgaricus and Streptococcus thermophilus. These bacteria work together in a symbiotic relationship to ferment milk into a thick, tangy probiotic food.

Key Points

Specific Bacterial Strains: Authentic Bulgarian yogurt is defined by the unique pairing of Lactobacillus bulgaricus and Streptococcus thermophilus.
Symbiotic Relationship: These two bacterial strains work together in a cooperative, mutually beneficial relationship to ferment milk into yogurt.
Natural Probiotic Powerhouse: The live cultures in Bulgarian yogurt are potent probiotics that support digestive health, enhance immunity, and may contribute to longevity.
Higher CFU Count: Bulgarian yogurt often boasts a significantly higher colony-forming unit (CFU) count than other yogurts, providing a more diverse probiotic effect.
Distinctive Flavor and Texture: The specific fermentation process, unlike strained yogurts like Greek yogurt, results in a uniquely thick, creamy, and tangy product.
Rich Nutritional Profile: The bacterial activity enhances the nutritional value of milk, producing vitamins, amino acids, and other beneficial compounds.
Supports Lactose Digestion: The presence of the beta-galactosidase enzyme from the live cultures helps break down lactose, potentially easing digestion for some.

The Iconic Bacterial Duo in Bulgarian Yogurt

Bulgarian yogurt, or kiselo mlyako, is renowned worldwide for its distinctively tart flavor and creamy texture. These characteristics are the direct result of a specific microbial fermentation process involving just two essential species of lactic acid bacteria. These two bacteria are Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus salivarius subspecies thermophilus, commonly abbreviated to L. bulgaricus and S. thermophilus.

The Symbiotic Partnership

The relationship between these two bacteria is a classic example of proto-cooperation, where both species benefit from working together. The fermentation process is a carefully orchestrated sequence of actions:

S. thermophilus starts the fermentation process by acting first. It produces acids that lower the pH of the milk, creating an ideal, slightly acidic environment for its partner to thrive.
L. bulgaricus, with its powerful proteolytic activity, then breaks down milk proteins into simpler amino acids and peptides. These compounds serve as a nutrient source that stimulates the growth of S. thermophilus.
The metabolic byproducts of this cooperative effort give Bulgarian yogurt its quintessential flavor profile.

This symbiotic collaboration allows for a faster and more efficient fermentation than either bacterium could achieve alone. The balance in the ratio of these bacterial strains is what makes Bulgarian yogurt taste and feel different from other yogurt varieties that may contain the same two bacteria but in different proportions.

Unveiling the Probiotic Power

The two signature bacteria in Bulgarian yogurt, L. bulgaricus and S. thermophilus, are recognized as powerful probiotics. When consumed, these live cultures contribute to a healthy gut microbiota, which is the community of microorganisms living in the digestive tract.

Health Benefits Linked to Bulgarian Yogurt Bacteria

Regular consumption of products containing these live, active cultures has been associated with numerous health advantages:

Improved Digestive Health: The probiotics assist in breaking down food, enhancing the absorption of nutrients, and alleviating common digestive discomforts like bloating, gas, and constipation.
Enhanced Immune Function: By maintaining a healthy balance of gut flora, these bacteria help to strengthen the body's immune system and reduce inflammation.
Potential for Longevity: Russian biologist and Nobel Prize-winner Élie Metchnikoff famously linked the high consumption of Bulgarian yogurt to the remarkable longevity of the peasant population in the country's mountainous regions.
Nutrient Creation: Beyond fermentation, the bacteria create valuable metabolic byproducts, including vitamins and amino acids, that further enrich the yogurt's nutritional profile.
Support for Lactose Digestion: L. bulgaricus and S. thermophilus produce the enzyme beta-galactosidase, which aids in the breakdown of lactose, potentially making it more digestible for lactose-intolerant individuals.

Bulgarian vs. Other Yogurts: A Microbial Comparison

While many yogurts contain L. bulgaricus and S. thermophilus, it is the unique symbiotic relationship and traditional, unstrained preparation of Bulgarian yogurt that truly sets it apart.

Comparison of Yogurt Types


Feature	Bulgarian Yogurt	Greek Yogurt	Regular Yogurt (US)
Starter Cultures	Primarily L. bulgaricus and S. thermophilus	Often the same two, sometimes with other added cultures	Varies by manufacturer, must contain L. bulgaricus and S. thermophilus
Straining Process	Not strained, contains all the whey	Strained to remove whey, resulting in a thicker consistency	Can be unstrained or lightly strained
Texture	Thick, creamy, and smooth due to fermentation	Very thick and rich due to straining	Can be thin, depending on fat content and processing
Flavor	Distinctive, tangy, and more tart than Greek yogurt	Milder and less tart than Bulgarian yogurt	Varies greatly, often sweeter due to added sugar
Probiotic Diversity	High CFU count and unique symbiotic effect	Contains probiotics, but often fewer strains and lower CFU than Bulgarian	Varies greatly by brand and added cultures

The Traditional Art of Making Bulgarian Yogurt

The traditional process for making Bulgarian yogurt is a testament to the symbiotic relationship of its bacteria and the importance of the environment. Authentic versions are made with fresh milk (often sheep's or buffalo's) and a starter culture from a previous batch. This heirloom method ensures the bacterial strains are propagated indefinitely. The milk is heated, then cooled to a specific temperature (around 42-45°C) before the starter is added. Incubation occurs without stirring, allowing the thermophilic bacteria to create the final product's specific set and texture. This careful process, combined with local environmental factors, results in the unique qualities of genuine Bulgarian yogurt.

Conclusion: The Enduring Legacy of Bulgarian Yogurt's Bacteria

Bulgarian yogurt is far more than just a food product; it is a rich cultural heritage, defined by the specific symbiotic relationship between Lactobacillus bulgaricus and Streptococcus thermophilus. This bacterial duo gives the yogurt its signature tart flavor and creamy texture, while also providing a wealth of probiotic benefits for digestive and immune health. The tradition of its production, based on a centuries-old fermentation process, results in a probiotic superfood with a uniquely high concentration of beneficial bacteria. For those seeking an authentic and powerful probiotic source, understanding what bacteria does Bulgarian yogurt have reveals the secret to its revered status as a true king among cultured dairy products.

For more in-depth scientific literature on the probiotic benefits of fermented dairy products, you can explore academic sources such as this study on the potential of yogurt in space travel.

Frequently Asked Questions

Bulgarian yogurt is made using a unique symbiotic culture of two bacterial species: Lactobacillus delbrueckii subspecies bulgaricus and Streptococcus salivarius subspecies thermophilus.

Yes, both Lactobacillus bulgaricus and Streptococcus thermophilus are considered powerful probiotics, which are live microorganisms that confer a health benefit to the host when consumed.

While many yogurts contain the same two species, it is the specific symbiotic relationship and often a higher concentration of these live, active cultures that distinguishes authentic Bulgarian yogurt. Many regular yogurts may contain other added bacteria, altering the final product's characteristics.

Yes, the live cultures in Bulgarian yogurt aid in digestion by helping to break down lactose and by contributing to a healthy balance of gut microbiota. This can help alleviate digestive issues like bloating and constipation.

While both are healthy, Bulgarian yogurt is often cited as having a more diverse and higher volume of probiotic bacteria per serving due to its unstrained nature, giving it a more potent probiotic effect compared to many Greek yogurts.

Some commercially available 'Bulgarian-style' yogurts may have additional bacteria added, such as Bifidobacterium. However, traditional Bulgarian yogurt is defined specifically by the presence of Lactobacillus bulgaricus and Streptococcus thermophilus.

The symbiotic relationship allows for faster fermentation and produces key compounds like lactic acid, which gives the yogurt its distinct tangy flavor and creamy texture. S. thermophilus prepares the environment for L. bulgaricus, which in turn provides nutrients for S. thermophilus.