What Are the Living Bacteria Found in Yogurt Called?

November 19, 2025 •

3 min read

Did you know that the Food and Agriculture Organization of the UN mandates specific bacteria for traditional yogurt production? The living bacteria found in yogurt called starter cultures are responsible for its tangy taste and thick texture.

Quick Summary

Yogurt's signature texture and flavor come from a symbiotic culture of bacteria, primarily Lactobacillus bulgaricus and Streptococcus thermophilus, which are also known as probiotics.

Key Points

Starter Cultures: The two essential bacteria in traditional yogurt are Lactobacillus bulgaricus and Streptococcus thermophilus.
Symbiotic Relationship: These two bacteria work together to efficiently ferment lactose into lactic acid, creating yogurt's characteristic texture and tartness.
Live Cultures vs. Probiotics: While all probiotic yogurts have live cultures, only specific strains in adequate amounts are considered probiotics and provide proven health benefits.
Added Probiotics: Many manufacturers include additional beneficial strains like Lactobacillus acidophilus and Bifidobacterium bifidum to enhance the health effects.
Health Benefits: Yogurt bacteria contribute to improved digestive function, immune system support, and nutrient absorption.
Labeling Matters: Look for the 'Live and Active Cultures' seal and check the ingredient list for specific probiotic strains to ensure you are getting the most beneficial product.

The Core Yogurt Cultures: Lactobacillus bulgaricus and Streptococcus thermophilus

Traditional yogurt relies on a symbiotic relationship between two specific bacterial species: Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus. These are required for products to be labeled as yogurt. This partnership drives the fermentation process.

The Symbiotic Partnership

This bacterial teamwork makes fermentation more efficient than either species working alone. Streptococcus thermophilus starts by consuming oxygen and breaking down milk proteins, creating a favorable environment for Lactobacillus bulgaricus. Lactobacillus bulgaricus then further breaks down proteins and produces amino acids, which stimulate Streptococcus thermophilus. Together, they ferment lactose into lactic acid, which thickens the milk and imparts yogurt's tart flavor.

Going Beyond the Starter Cultures: Probiotics and Live Cultures

While essential for yogurt making, L. bulgaricus and S. thermophilus may not survive the digestive tract in sufficient numbers to provide significant health benefits. This highlights the difference between 'live cultures' and 'probiotics'.

Live Cultures: This broad term refers to any live microorganisms in food, including the starter cultures. Yogurts with probiotics contain live cultures, but not all yogurts with live cultures have beneficial probiotics in adequate amounts.
Probiotics: These are specific live microorganisms that, in sufficient quantities, provide a health benefit. Many yogurts include additional probiotic strains to offer specific health advantages.

Common Probiotic Strains Added to Yogurt

Lactobacillus acidophilus: Known for surviving the digestive system, it may support gut health and lactose digestion.
Bifidobacterium bifidum: Often added to support digestive health and the gut microbiome.
Lactobacillus casei: This strain can withstand digestion and is linked to digestive and immune health.

The Health Benefits of Yogurt Bacteria

Consuming yogurt with live and active cultures or added probiotics offers numerous health benefits, stemming from their activity in the digestive system and interaction with the gut microbiome.

Enhanced Digestive Function

Yogurt's lactic acid bacteria ferment lactose, making it easier for some individuals with lactose intolerance to consume. Probiotics can also help balance the gut microbiome, potentially easing symptoms like diarrhea, bloating, and constipation.

Immune System Modulation

Probiotics help support a healthy immune response by fostering a balanced gut microbiome, a key component of the immune system.

Improved Nutrient Absorption

Some probiotics may enhance the absorption of nutrients like calcium and certain vitamins.

Potential Heart and Weight Benefits

Regular yogurt consumption may be linked to improved cardiovascular health, including lower blood pressure and cholesterol. The protein in yogurt, particularly Greek yogurt, can contribute to satiety and aid weight management.

Comparison of Yogurt Types and Their Bacterial Content

The bacterial content in different yogurts varies, affecting texture and probiotic potential. Check labels for the 'Live and Active Cultures' seal and specific strains.


Feature	Regular Yogurt	Greek Yogurt	Probiotic-Enriched Yogurt
Starter Cultures	Yes (L. bulgaricus, S. thermophilus)	Yes (L. bulgaricus, S. thermophilus)	Yes (L. bulgaricus, S. thermophilus)
Additional Probiotics	Often none, unless stated	Sometimes added	Explicitly contains added, beneficial probiotic strains
Straining	Not strained	Strained to remove whey	Straining varies by type
Bacterial Count (CFU)	Generally high enough to meet minimum standard (>10^6 CFU/g)	Often higher due to concentration	Contains guaranteed high count of specific strains
Texture	Thinner, more liquid	Thicker, creamier	Varies depending on straining

How to Choose a Yogurt with Beneficial Bacteria

When selecting yogurt, look beyond the word 'probiotics'. Consider these factors:

The 'Live and Active Cultures' Seal: This voluntary seal from the National Yogurt Association indicates the product had at least 100 million live bacteria per gram at manufacturing.
Ingredient List: Look for specific strains like L. acidophilus or B. bifidum listed alongside the starter cultures.
Storage Conditions: Proper refrigeration is crucial as live cultures are sensitive to heat.

Conclusion: Understanding Your Yogurt Cultures

The living bacteria in yogurt, known as live cultures, are primarily the symbiotic starter strains Lactobacillus bulgaricus and Streptococcus thermophilus, vital for fermentation and the resulting taste and texture. Probiotics, specific added bacterial strains, offer targeted health benefits, particularly for the gut microbiome. By understanding these bacteria and reading labels, consumers can make informed choices for their health. For more information on incorporating beneficial microbes through foods like yogurt, consult Harvard Health's guide on probiotics.

Please note: This information is for educational purposes only and not medical advice. Consult a healthcare professional before dietary changes.

Frequently Asked Questions

The two main starter cultures used for traditional yogurt are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus salivarius subsp. thermophilus.

All probiotics are a type of live culture, but not all live cultures are probiotics. The term 'probiotic' is reserved for live microorganisms that provide a health benefit when consumed in sufficient quantities.

The bacteria ferment lactose into lactic acid, which can make yogurt easier to digest for people with lactose intolerance. Probiotics also help maintain a healthy balance of bacteria in the gut.

No. While all yogurt made with live cultures contains the starter bacteria, only yogurts with specific, added strains in adequate amounts can be considered probiotic.

Yogurt is typically pasteurized before the cultures are added. Manufacturers must list 'live and active cultures' for the bacteria to be present in the final product.

It is a seal from the National Yogurt Association that ensures the product contained a certain minimum level of live bacteria per gram at the time of manufacture.

Greek yogurt is thicker because it has been strained to remove excess whey and liquid, resulting in a more concentrated product with higher protein and, often, a higher bacterial count.