Probiotic Strains Explained: Sources and Benefits

How Probiotic Strains Are Named

Probiotics are identified by a three-part name: genus, species, and strain. For example, *Lacticaseibacillus rhamnosus* GG (formerly *Lactobacillus rhamnosus* GG) is a well-known strain. The strain designation is an alphanumeric code (often a brand or laboratory identifier) that distinguishes one specific microbe from others of the same species. Some names also include a subspecies level, such as *Bifidobacterium animalis* subsp. *lactis* BB-12.

This naming system matters because health benefits are strain-specific. A benefit observed with one strain may not hold for another, even within the same species. For example, the effects of *Lacticaseibacillus rhamnosus* GG are studied as a distinct strain, not as a general property of all lactobacilli.

When you see a probiotic label, look for the full three-part name. Foods and supplements that list only the genus (such as “contains Lactobacillus”) or only the species without a strain designation are less precise about what you are getting.

• Genus: a broad group, such as *Lactobacillus* or *Bifidobacterium*.
• Species: a more specific group, such as *Lactobacillus acidophilus*.
• Strain: a genetic variant, such as *Lactobacillus acidophilus* NCFM.
• After the 2020 reclassification, many former *Lactobacillus* species were moved to new genera, e.g., *Lactobacillus rhamnosus* became *Lacticaseibacillus rhamnosus*.

Sources: National Institutes of Health, Office of Dietary Supplements, PureMed

Common Probiotic Genera and Their Roles

Probiotic products and fermented foods contain a variety of microorganisms. The genera most often associated with probiotics are *Bifidobacterium* and members of the family Lactobacillaceae (which includes former *Lactobacillus* species). Other bacteria used in probiotic products include *Streptococcus*, *Enterococcus*, *Escherichia*, and *Bacillus*. There is also one important yeast genus, *Saccharomyces*.

These organisms work through several mechanisms. Some non-specifically inhibit the growth of harmful bacteria, others produce vitamins or other beneficial compounds, and some have strain-specific effects, such as modulating the immune system. The effect you get from a fermented food or supplement depends on the specific strain it contains.

• Bifidobacterium: commonly found in dairy and the human gut.
• Lactobacillaceae: the family that includes species like *Lactiplantibacillus plantarum* and *Lacticaseibacillus rhamnosus*.
• Saccharomyces boulardii: a yeast used in some probiotic supplements.
• Bacillus: a spore-forming genus, often used in supplements.
• Streptococcus thermophilus and *Lactobacillus delbrueckii* subsp. *bulgaricus* are traditional yogurt starters, occasionally serving as probiotics.

Sources: National Institutes of Health, Office of Dietary Supplements, PureMed, Foods (MDPI)

Strains in Yogurt and Fermented Dairy

Yogurt is probably the most familiar fermented food. Traditional yogurt is made with two starter cultures, *Lactobacillus delbrueckii* subsp. *bulgaricus* and *Streptococcus thermophilus*. Many commercial yogurts also add live probiotic strains, such as *Bifidobacterium animalis* subsp. *lactis* or *Lactobacillus acidophilus*, to boost their potential health effects.

Kefir, a fermented dairy drink, typically contains a more complex mix of bacteria and yeast. Research has identified *Lactobacillus kefiri*, *Lactobacillus kefiranofaciens*, strains of *Lactobacillus plantarum* and *Lactobacillus paracasei*, along with *Lactococcus lactis*, *Streptococcus thermophilus*, and various yeasts. The exact makeup can vary by producer.

Certain cheeses, such as Swiss, Gouda, cheddar, and cottage cheese, may also contain probiotics, but the amount and type vary widely. Not all cheeses are made with live cultures—some are heat-treated after fermentation, which kills the bacteria.

Fermented dairy products like yogurt may be easier to digest for some people with lactose intolerance, because the lactic acid bacteria produce lactase and reduce lactose content. This effect, however, depends on the product and the individual.

• Yogurt with live cultures often contains *Bifidobacterium animalis* subsp. *lactis*, *Lactobacillus acidophilus*, and *Streptococcus thermophilus*.
• Kefir can contain lactobacilli and bifidobacteria along with yeast species.
• Cheese may have probiotics but strains are often not defined on labels.
• Heat-treated fermented dairy products do not contain live microbes.

Sources: National Institutes of Health, Office of Dietary Supplements, Cleveland Clinic, U.S. Dairy, Journal of Advanced Pharmaceutical Technology & Research, Nutrients (MDPI)

Strains found in fermented vegetables

Fermented vegetables—like sauerkraut, kimchi, and pickles—naturally harbor a variety of lactic acid bacteria (LAB), including *Lactobacillus*, *Leuconostoc*, *Weissella*, *Pediococcus*, *Lactococcus*, and *Enterococcus*. These bacteria are responsible for the sour flavor and the preservation of the vegetables.

However, not all fermented vegetables deliver probiotic strains when you eat them. Many commercial products are pasteurized, canned, or heat-treated after fermentation, which kills the live microbes. Even fresh, unpasteurized sauerkraut and kimchi may contain live LAB, but they do not meet the definition of probiotics unless the specific strains have been identified and shown to confer a health benefit.

Kimchi, for example, often contains *Lactobacillus*, *Leuconostoc*, and *Weissella* species. *Lactiplantibacillus plantarum* is a common species in fermented vegetables and has been studied in laboratory settings for possible health-promoting properties. Yet these are not necessarily the same as the clinically validated strains found in yogurt or supplements.

Kombucha, a fermented tea, contains live bacteria and yeast. But scientists at ISAPP and the Canadian Digestive Health Foundation point out that kombucha’s microbes are usually not defined to strain level, and no specific health benefits have been demonstrated. So while kombucha is a source of live microbes, it is not typically a “probiotic” food in the strict sense.

• Sauerkraut (unpasteurized) and kimchi contain live LAB but generally do not meet probiotic criteria.
• Miso is a fermented soybean paste that can contain live microbes, but strain identity is often unclear.
• Kombucha is a fermented tea with live bacteria and yeast; probiotic status is debated.
• Processed fermented foods like most commercial pickles, beer, and sourdough bread do not contain live microbes at consumption.

Sources: Cleveland Clinic, Journal of Advanced Pharmaceutical Technology & Research, Foods (MDPI), International Scientific Association for Probiotics and Prebiotics, Canadian Digestive Health Foundation

Evidence for Strain-Specific Benefits

The most reliable evidence for probiotic benefits comes from studies of specific strains, not entire species. Scientific reviews indicate that certain strains have been studied for conditions such as diarrhea and lactose digestion, but findings for one strain do not automatically apply to another.

For example, some strains of *Bifidobacterium* and *Lactobacillus* have been studied for their potential to improve lactose digestion, as fermented dairy products can contain bacteria that produce lactase. However, evidence for specific benefits like constipation relief or immune support is often strain-specific and may be limited.

If you are considering a probiotic for a particular health concern, it’s important to look for products that list the specific strain and to consult a healthcare professional for personalized advice.

• Strain-specific effects are a key principle: benefits seen with one strain may not occur with another.
• Fermented dairy may be easier to digest for some people with lactose intolerance due to bacterial lactase production.
• For other health benefits, evidence is continually evolving, and not all strains have been studied for all conditions.

Sources: National Institutes of Health, Office of Dietary Supplements, Nutrients (MDPI)

Safety Considerations by Strain and Population

Probiotics are generally safe for healthy people, but certain strains and populations require caution. People with weakened immune systems, those who are severely ill, and preterm infants with central venous catheters should generally avoid probiotics unless a healthcare provider advises otherwise.

For most people, the main side effects are mild gas or bloating, which usually subside. If you are considering a probiotic for a medical condition, talk with your doctor. This is especially important for infants, elderly people, and anyone with a compromised gut barrier.

There is no official safety classification for every strain on the market, so use your judgment and rely on reputable sources. If you have a chronic condition or take immunosuppressive drugs, probiotic supplements are not necessarily right for you.

• Healthy adults can typically consume fermented foods and probiotic supplements without issues.
• Preterm infants, those who are severely ill, and individuals with central venous catheters should avoid probiotics unless a clinician says it is safe.
• Some *Lactobacillus* and *Saccharomyces* strains have caused rare infections in immunocompromised individuals.
• If you are pregnant or nursing, discuss with your provider before using probiotic supplements.

Sources: National Institutes of Health, Office of Dietary Supplements, PureMed

Identifying Strains on Food Labels

When you pick up a food or supplement, the label should tell you exactly which strains are inside. Look for the full three-part name. For example, “*Bifidobacterium animalis* subsp. *lactis* BB-12” is a specific strain; “*Bifidobacterium*” alone is not.

In the United States, product labels are required to list the genus and species, but not necessarily the strain. If the strain is not listed, you cannot assume it has the same properties as a named, studied strain.

Colony-forming units (CFU) indicate the number of viable cells. A higher CFU does not automatically mean a better product; effectiveness depends on the strain and formulation. Heat-treated foods that list “live cultures” may not actually contain them—check for the word “live” and avoid products that have been pasteurized after fermentation.

• Look for a three-part name: genus, species, and strain (e.g., *Lactobacillus acidophilus* NCFM).
• Be wary of labels that say “probiotic” but do not list specific strains.
• CFU counts can vary, and a higher count is not always better.
• Some foods have “live active cultures” but are not probiotic because the strains are not defined or beneficial.

Sources: National Institutes of Health, Office of Dietary Supplements, PureMed