Probiotics are live microorganisms that benefit your health when you consume enough of them. They come in two main forms: as specific bacterial or yeast strains sold in supplements, and as naturally occurring cultures in fermented foods like yogurt, kefir, and kimchi. The most common probiotic organisms fall into a handful of groups, each with different strengths.
The Most Common Bacterial Probiotics
The vast majority of probiotic products use bacteria from two large families: Lactobacillus (and its recently renamed relatives) and Bifidobacterium. You’ll see these names on virtually every supplement label and in the ingredient lists of probiotic-fortified foods.
Within the Lactobacillus family, the most widely used species include:
- L. acidophilus — one of the most recognizable probiotic names, found in yogurt and supplements for decades
- L. rhamnosus GG (LGG) — one of the most extensively studied strains in the world, with strong evidence for preventing diarrhea during antibiotic use
- L. casei — studied specifically for reducing severe diarrhea caused by the bacterium C. difficile
- L. reuteri — commonly found in supplements marketed for digestive comfort and infant colic
- L. plantarum — often used in fermented vegetables and studied for gut barrier support
- L. bulgaricus — a traditional yogurt-making culture paired with Streptococcus thermophilus
In the Bifidobacterium family, commonly used species include B. animalis (often marketed under the nickname “Bifidus regularis” in commercial yogurts), B. longum, and B. infantis. Bifidobacteria are among the very first microbes to colonize an infant’s gut and play a foundational role in early immune development. B. infantis is uniquely equipped to break down sugars found in breast milk, which is why it shows up in infant-specific probiotic formulas.
Scientists restructured the Lactobacillus genus in 2020, giving many species new formal names. L. rhamnosus is now technically Lacticaseibacillus rhamnosus, for example. But all the new genus names still start with “L,” so the abbreviated versions on product labels haven’t changed much. You don’t need to memorize the new taxonomy to shop for probiotics.
Yeast-Based Probiotics
Not all probiotics are bacteria. Saccharomyces boulardii is a non-harmful yeast and the most widely used probiotic outside the bacterial world. It works differently from bacterial strains: once in your gut, it forms a protective layer along the intestinal wall that helps block harmful organisms from attaching. It also produces enzymes that break down specific toxins made by C. difficile, a dangerous bacterium that can take over the gut after antibiotic treatment.
S. boulardii is one of the two probiotics with the strongest track record for preventing antibiotic-associated diarrhea, alongside L. rhamnosus GG. Effective doses in studies ranged from 5 to 40 billion colony-forming units (CFUs) per day. Because it’s a yeast rather than a bacterium, antibiotics don’t kill it, which makes it a practical choice to take alongside antibiotic prescriptions.
Spore-Forming Probiotics
Bacillus coagulans is a different kind of probiotic bacterium that forms protective spores. These spores act like armor: they survive high temperatures, stomach acid, and long storage times without refrigeration. Once the spores reach your small intestine, they activate and begin working, helping digest carbohydrates and producing lactic acid much like traditional Lactobacillus strains do.
This resilience makes Bacillus coagulans popular in shelf-stable products like granola bars, protein powders, and other processed foods where traditional probiotics wouldn’t survive manufacturing. It has been designated as Generally Regarded as Safe (GRAS) and studied for its effects on chronic diarrhea, irritable bowel syndrome, and constipation. It also produces a natural antimicrobial compound called coagulin that may help keep harmful bacteria in check.
Fermented Foods With Live Cultures
You don’t need a supplement to consume probiotics. Fermented foods are the original source, and many contain diverse communities of beneficial microbes rather than a single isolated strain.
- Yogurt — contains L. bulgaricus and Streptococcus thermophilus as its base cultures, and many brands add additional strains like L. acidophilus or B. animalis
- Kefir — a tangy, drinkable fermented milk with a broader range of bacterial and yeast species than yogurt
- Kimchi — Korean fermented vegetables rich in Lactobacillus species, along with vitamins and fiber
- Sauerkraut — fermented cabbage that contains live Lactobacillus cultures when sold unpasteurized (heat-treated versions in shelf-stable jars typically have no live organisms)
- Some pickles — cucumbers fermented in saltwater brine (not vinegar) contain live cultures
- Kombucha — a fermented tea containing bacteria and yeast
The key detail with fermented foods is that the product must contain live cultures at the time you eat it. Pasteurization, which many shelf-stable products undergo, kills the organisms. Look for “contains live and active cultures” on the label, and check whether the product is refrigerated.
Prebiotics Are Not the Same Thing
You’ll often see prebiotics mentioned alongside probiotics, and it’s worth knowing the difference. Prebiotics are not living organisms. They’re specific types of fiber that feed the beneficial bacteria already in your gut. Fructo-oligosaccharides (FOS) and inulin, found naturally in foods like garlic, onions, bananas, and asparagus, are the best-studied prebiotics. They selectively boost populations of Bifidobacterium in the gut. Some products combine both prebiotics and probiotics, marketed as “synbiotics.”
What to Know About Supplements
Probiotic supplements in the U.S. are regulated as dietary supplements, not drugs. Under the Dietary Supplement Health and Education Act of 1994, manufacturers are responsible for ensuring their products are safe and accurately labeled before selling them. But the FDA does not approve probiotic supplements for effectiveness the way it approves medications. It can only take action against a product after it reaches the market if it’s found to be adulterated or mislabeled.
This means quality varies widely between brands. When choosing a supplement, look for products that list the full strain designation (for example, “Lactobacillus rhamnosus GG” rather than just “Lactobacillus”), a CFU count guaranteed through the expiration date rather than just “at time of manufacture,” and clear storage instructions.
Refrigerated vs. Shelf-Stable Products
Whether a probiotic needs refrigeration depends on how it’s made. Probiotic foods like yogurt, kefir, and fermented milk rely on cold temperatures combined with their natural acidity to keep organisms alive but dormant during storage. These always need refrigeration.
Most supplement capsules and tablets are freeze-dried, which drops their moisture content so low that the organisms essentially go into suspended animation. These products can often be stored at room temperature without losing potency. Spore-forming probiotics like Bacillus coagulans are the most shelf-stable of all, surviving heat and humidity that would kill other strains. If a supplement label says to refrigerate it, follow those instructions. The manufacturer chose strains or packaging that require cold storage to maintain the promised CFU count through expiration.