Exploring the Origins: Where is L. casei found naturally?

December 29, 2025 •

4 min read

In the early 1900s, scientists first discovered the adaptable bacterium L. casei (now Lacticaseibacillus casei) in cheese, a discovery that gave the microbe its name, from the Latin word casei meaning “of cheese”. This initial discovery hints at its diverse natural origins beyond commercial supplements, prompting a deeper look into where this beneficial probiotic can be found in the wild.

Quick Summary

An adaptable bacterium, L. casei is naturally present in several environments, including fermented dairy products, aged cheeses, fermented vegetables, and the human gastrointestinal tract. Specific food sources include raw milk, naturally ripened olives, and sauerkraut. Its ability to thrive in various habitats explains its widespread presence and role in different fermentations.

Key Points

Diverse Habitats: L. casei is a highly adaptable microbe found in multiple natural environments, including dairy products, plants, and the human gut.
Aged Cheese: It is a key non-starter bacterium in the natural ripening of hard, long-aged cheeses like Parmigiano Reggiano and Grana Padano, originating from raw milk.
Fermented Vegetables: Natural sources include traditionally fermented vegetables such as sauerkraut and olives, where it thrives in the brine.
Human Microbiome: The bacterium is a native component of the human gastrointestinal and urogenital microflora, capable of adapting to the harsh digestive tract environment.
Natural vs. Added Cultures: Unlike many commercial probiotic products where cultures are added, the L. casei in these traditional foods occurs naturally through fermentation.

A Nomadic Microbe with Diverse Habitats

Lacticaseibacillus casei is often described by microbiologists as a “nomadic” bacterium due to its impressive ability to thrive in many different environments, from plant surfaces to the human gut. While commercially produced probiotic products are a well-known source, understanding where L. casei is found naturally can provide a more holistic view of its role in both the ecosystem and our diet. Its presence is primarily linked to the process of natural fermentation, where it plays a key role in developing flavor and acting as a biopreservative.

Natural Sources in Dairy

Perhaps the most well-documented natural source of L. casei is fermented dairy. Its very name, derived from the Latin for cheese, speaks to its strong association with these products. The strains of the L. casei group are frequently found in raw milk, especially those used for hard, cooked, long-ripened cheeses.

Aged and Ripened Cheeses

In cheeses like Italy's Parmigiano Reggiano and Grana Padano, L. casei starts out in low numbers in the raw milk but flourishes during the long ripening process. It is considered a non-starter lactic acid bacterium (NSLAB), meaning it contributes to the maturation and complex flavor profile rather than the initial acidification. The cheese environment, with its changing pH and nutrient availability, selects for resilient bacteria like L. casei.

Fermented Milk and Yogurt

Some traditional or naturally fermented yogurts and yogurt-like drinks can also contain L. casei. The presence of live and active cultures is crucial, as pasteurization can eliminate beneficial bacteria. Checking product labels for terms like “live and active cultures” or seeking out traditionally made products is key for consumers looking for natural sources. In fermented milks, L. casei is capable of persisting throughout the shelf-life, ensuring the product retains its probiotic content.

Plant-Based Origins

Beyond dairy, L. casei is also a common resident of plant surfaces and is isolated from a variety of naturally fermented vegetable products. Strains originating from plant environments carry specific genes that help them utilize plant-based sugars for survival.

Naturally Fermented Olives: Sicilian green olives that undergo natural fermentation are known to contain L. casei as a dominant species. The brine creates an environment conducive to its growth. This is different from olives processed with chemicals or vinegar, which would kill the natural microbes.
Sauerkraut: Authentic, traditionally made sauerkraut, which ferments shredded cabbage in its own brine, is another excellent source of L. casei. Again, mass-produced versions that use vinegar or pasteurization will not contain these live cultures. The presence of L. casei and other lactic acid bacteria contributes to its characteristic sour taste and probiotic benefits.
Sourdough Bread: While not a primary source, some sourdough breads rely on natural fermentation that can include lactic acid bacteria like L. casei, along with yeast, contributing to the distinct flavor.

In the Human and Animal Microbiota

L. casei is a natural part of the microflora found in the gastrointestinal tract, mouth, and urogenital tracts of humans and animals. Its ability to tolerate and adapt to the harsh conditions of the gut, such as stomach acid and bile, allows it to survive transit and interact with the resident microbiota. Comparative genomic analyses have even highlighted distinct genetic traits in strains from dairy, plant, and human origins, underscoring its impressive adaptability to different ecological niches. While present in the gut naturally, its population can be influenced by diet and other factors.

Natural vs. Commercial Sources

When seeking out L. casei, it's important to understand the difference between natural food sources and commercially produced supplements or fortified foods. The key distinction lies in how the probiotic is introduced and whether it occurs naturally during fermentation.


Feature	Natural Food Sources	Commercial Supplements/Fortified Foods
Origin of L. casei	Naturally present in raw ingredients and selected by the fermentation process	Specific strains are intentionally added during manufacturing
Microbial Diversity	Often contains a wide variety of microbial species, creating a complex community	Usually contains a limited number of specific, lab-cultivated strains
Processing	Involves traditional fermentation methods; raw or unpasteurized to preserve live cultures	Pasteurized products with added cultures, or encapsulated/freeze-dried bacteria
Guarantee of Live Cultures	Varies; depends on the food's production and storage conditions	High guarantee of live, viable bacteria as measured by CFUs
Example	Naturally aged cheeses, real sauerkraut, raw milk kefir	Probiotic drinks (Yakult), capsules, fortified yogurts (e.g., DanActive)

For consumers looking for the most natural way to ingest L. casei, traditionally fermented foods are the ideal choice. However, commercially added cultures and supplements offer a consistent and high-dose option that can be a reliable part of a balanced diet. Ultimately, both offer a route to increasing one's intake of this beneficial microbe, but their origins and microbial context are distinct.

Conclusion

The ubiquitous and highly adaptable nature of L. casei makes it a fascinating inhabitant of a wide array of natural environments. Its presence spans the dairy realm, particularly in aged cheeses and raw milk products, and extends to plant-based fermentations such as naturally pickled olives and sauerkraut. Furthermore, it plays a native role in the complex microbial communities of the human and animal digestive systems. By consuming traditionally fermented foods, one can access this beneficial bacterium as part of a rich, diverse microbial community, offering a natural and historical method of supporting gut health.

Sources and Benefits of the Lacticaseibacillus casei Group

Frequently Asked Questions

The primary difference lies in the origin and process. In natural sources like aged cheese or sauerkraut, L. casei develops organically during fermentation. In commercial products, specific strains are intentionally added to a food item or encapsulated in a supplement.

No. L. casei is particularly prominent in aged, hard cheeses made from raw milk, where it becomes a dominant microbe during the long ripening process. Most mass-produced or pasteurized cheeses may not contain it, and different strains and microbes contribute to various cheese types.

No. Only pickles made through natural, salt-brine fermentation, such as some traditional deli-style pickles, contain live cultures. Pickles preserved in vinegar are not naturally fermented and lack live probiotic bacteria.

Yes. Beyond dairy, natural L. casei can be found in fermented plant-based products, including naturally fermented green olives, sauerkraut, and some artisanal sourdough breads.

Some L. casei strains are particularly resilient, with adaptations that allow them to survive the harsh, acidic conditions of the stomach and bile in the intestines. This enables them to reach the gut and exert their beneficial effects on the microbiome.

No. The presence of live L. casei depends on the yogurt's specific culture blend and whether it has been heat-treated after fermentation. Look for products specifically labeled with "live and active cultures" and check the ingredient list for the specific strains used.

Yes. While L. casei is a natural part of the gut microbiota, consuming foods rich in this and other beneficial bacteria, along with prebiotic fiber to feed them, can help support and influence the composition and diversity of your gut flora over time.