Probiotics: What They Are, How They Work, and What They’re Used For
What probiotics are, how they work, how they differ from prebiotics and postbiotics, which foods contain them, and what to know about supplements and CFU.
Key takeaways
- Probiotics are live microorganisms that, in adequate amounts, may provide health benefits.
- The best-known probiotics include certain bacteria and yeasts.
- Different probiotic strains can have different effects.
- Not all fermented foods are automatically sources of probiotics.
- A higher CFU count does not automatically mean a probiotic is more effective.
In this article
- What are probiotics?
- How do probiotics work in the body?
- What is the difference between probiotics, prebiotics, and postbiotics?
- What are the most common types of probiotics?
- What probiotic benefits are supported by research?
- Which foods contain probiotics?
- What is the difference between fermented foods and probiotic supplements?
- How do we choose a probiotic supplement, and what does CFU mean?
- When and how should probiotics be taken?
- Can probiotics cause side effects, and who should be cautious?
- Frequently asked questions
- Final thoughts
Probiotics are live microorganisms that, when consumed in adequate amounts, may provide health benefits. They are most commonly certain types of bacteria, although some yeasts can also function as probiotics.
They can be found in certain foods and dietary supplements. Probiotics are studied primarily for their effects on the gastrointestinal tract, gut microbiota, and digestion, although research also covers other areas of health.
One distinction is important from the beginning: not all bacteria are probiotics, and not all probiotics have the same effects. Potential benefits can depend on the microorganism, strain, amount, and reason it is being used.
What are probiotics?
Probiotics are live microorganisms that, when administered in adequate amounts, can provide a health benefit. This is the widely accepted scientific definition of a probiotic.
The term “microorganism” refers to organisms so small that they cannot be seen with the naked eye. In the case of probiotics, these are most often bacteria, although certain yeasts are also used as probiotics.
Probiotics are often described simply as “good bacteria,” but this description is incomplete. Not all probiotics are bacteria, and simply finding live bacteria in a product does not automatically make them probiotic.
In practical terms, a probiotic is a live microorganism associated with a specific health benefit when provided in an adequate amount. The presence of live microorganisms alone is not enough to establish that benefit.
How do probiotics work in the body?
Much of the research on probiotics focuses on the gastrointestinal tract. This is where a large and diverse community of microorganisms, known as the gut microbiota, normally lives.
Depending on the microorganism, probiotics may interact with microbes already present in the intestine, the intestinal barrier, and the immune system. Some can also produce substances that influence the environment within the digestive tract.
Probiotics should not be viewed simply as bacteria that enter the intestine and “replace bad bacteria.” The relationship between probiotics, the existing microbiota, and the human body is considerably more complex.
A probiotic also does not necessarily need to permanently colonize the intestine to have an effect. Some microorganisms may exert effects while they are passing through the gastrointestinal tract.
What is the difference between probiotics, prebiotics, and postbiotics?
These terms are frequently used together, but they describe different things. Understanding the distinction helps avoid treating them as interchangeable categories.
| Term | What it means |
|---|---|
| Probiotics | Live microorganisms that, in adequate amounts, may provide health benefits |
| Prebiotics | Components in foods, particularly certain types of fiber, that are used by beneficial microorganisms in the gut and may support their activity |
| Postbiotics | Preparations containing inactivated microorganisms and/or their components that may provide health benefits |
Although the names are similar, probiotics, prebiotics, and postbiotics are different categories with different characteristics. Depending on the product, they may be present separately or combined in foods and supplements.
What are the most common types of probiotics?
Probiotic supplements may contain a single microorganism or combinations of several microorganisms. Common examples include bacteria from groups such as Lactobacillus, Bifidobacterium, and certain Bacillus species, as well as the yeast Saccharomyces boulardii.
The general name of a microorganism is not always enough to tell us what effects it may have. A probiotic label may identify the genus, species, and, in well-characterized products, the specific strain.
The strain can be particularly important because microorganisms belonging to the same species may have different properties. If a benefit was demonstrated in a study using one specific strain, that result should not automatically be attributed to every microorganism belonging to the same species.
What probiotic benefits are supported by research?
Probiotics have been studied for many health conditions, particularly those involving the gastrointestinal tract. Research has found promising results and, in some situations, benefits for particular products and strains, but the effects are not the same across all probiotics.
One extensively studied area is antibiotic-associated diarrhea. Some analyses of clinical trials have found a lower risk in certain populations when particular probiotic preparations were used.
Probiotics have also been investigated for other types of diarrhea and a range of gastrointestinal conditions. For many uses, however, there is still not enough evidence to recommend one strain, dose, or duration for everyone.
This is why a benefit demonstrated for one probiotic product should not automatically be extended to every supplement labeled “probiotic.” Evidence is often specific to the microorganism, strain, dose, population, and clinical context.
Which foods contain probiotics?
Some common foods may contain live microorganisms. Yogurt and kefir are among the best-known examples, while live cultures may also be present in certain cheeses and fermented vegetables.
Products within the same food category are not necessarily identical. Processing, pasteurization, and heat treatment can determine whether microorganisms remain alive in the product when it is consumed.
| Food | What to know |
|---|---|
| Yogurt | One of the most common sources of live cultures; check the product label |
| Kefir | Commonly contains bacteria and yeasts |
| Some cheeses | May retain live microorganisms depending on the type and processing |
| Sauerkraut | May contain live microorganisms if it has not been pasteurized or heat treated |
| Fermented pickles | May contain live microorganisms when fermented in brine and not pasteurized |
| Olives in brine | Some naturally fermented products may contain live microorganisms |
| Kimchi | May contain live microorganisms if it has not been pasteurized or heat treated |
| Miso | Whether live microorganisms remain depends on processing and heat treatment |
| Tempeh | Produced through fermentation of soybeans |
| Natto | Fermented soybean food made using Bacillus subtilis var. natto |
Not every food in this table is automatically probiotic. For example, pickles preserved in vinegar are different from vegetables naturally fermented in brine, and pasteurization can inactivate live microorganisms.
For yogurt and kefir, the label may provide information about the cultures used. Even then, the presence of live cultures does not automatically mean the product has a specific clinically demonstrated probiotic benefit.
What is the difference between fermented foods and probiotic supplements?
Fermented foods are produced with the help of microorganisms. This does not automatically mean those microorganisms remain alive in the final product or that they have a demonstrated probiotic benefit.
Probiotic supplements, by contrast, are formulated to provide specific live microorganisms. A well-characterized product should make it possible to identify the microorganisms it contains and the amount being provided.
| Fermented foods | Probiotic supplements |
|---|---|
| Are foods | Are dietary supplements |
| May contain live microorganisms | Are formulated to provide live microorganisms |
| Microorganisms can vary | May contain specific strains |
| Are not automatically probiotics | May contain microorganisms with studied effects |
| Provide the nutrients present in the food | Composition depends on the product formula |
Fermented foods can still be part of a varied diet even when they do not meet the strict scientific definition of a probiotic. Fermentation can change a food’s taste, texture, and composition, and these characteristics are separate from whether a specific probiotic benefit has been demonstrated.
How do we choose a probiotic supplement, and what does CFU mean?
Probiotic labels commonly use the abbreviation CFU, or Colony-Forming Units. This unit is used to express the amount of viable microorganisms in a product.
For example, 1 billion CFU means one billion colony-forming units, while 10 billion CFU means ten billion. A larger number, however, does not automatically mean that the supplement is better or more effective.
| What to check | Why it matters |
|---|---|
| Genus and species | Shows which microorganism the product contains |
| Strain | Allows more precise identification of the microorganism studied |
| CFU | Indicates the declared number of viable microorganisms |
| Serving Size | Shows how many capsules or how much product makes up one serving |
| Expiration date | Important because microorganisms need to remain viable |
| Storage instructions | Shows whether refrigeration or room-temperature storage is required |
| Other Ingredients | Shows what else is included in the supplement |
More CFU does not automatically mean a better probiotic. The amount needs to be considered together with the microorganism, strain, and the quantity that has been studied for a particular purpose.
Some probiotics require refrigeration, while others are formulated to remain stable at room temperature. The storage conditions listed by the manufacturer should be followed to help preserve product quality.
When and how should probiotics be taken?
There is no universal rule stating that every probiotic should be taken in the morning, at night, before meals, or after meals. The appropriate timing can depend on the microorganism, product formulation, and reason the probiotic is being used.
The same applies to how long probiotics should be taken. Some products have been studied for relatively short periods, while others have been investigated for longer durations and in different clinical situations.
When probiotics are used during antibiotic treatment, both the microorganism and timing may be relevant. The general advice to take “any probiotic” during or after antibiotics is too broad because probiotic products are not interchangeable.
Product-specific instructions should therefore be considered rather than assuming that all probiotics are used in the same way. When probiotics are being considered for a medical reason, a doctor or pharmacist can help evaluate the specific product and context.
Can probiotics cause side effects, and who should be cautious?
In healthy people, probiotics are generally well tolerated, and reported adverse effects are often mild. Some people may experience gas, bloating, or temporary changes in bowel habits.
The fact that a product contains microorganisms considered beneficial does not mean it is risk-free for everyone. Serious infections have been reported in certain vulnerable groups, including people with severe illnesses, compromised immune systems, and some premature infants.
People with serious medical conditions, immunosuppression, or other circumstances that increase the risk of infection should discuss probiotic use with a healthcare professional. Probiotic supplements should not be used as a replacement for necessary medical treatment.
Product quality also matters because identity, viable microorganism count, manufacturing, and storage can affect what the consumer actually receives. Following storage instructions and using well-characterized products are therefore important practical considerations.
Frequently asked questions
Are probiotics bacteria?
Many probiotics are bacteria, but not all of them are. Certain yeasts, including Saccharomyces boulardii, are also used as probiotics.
Are probiotics and prebiotics the same thing?
No, because probiotics are live microorganisms while prebiotics are components that certain microorganisms can use. Some types of dietary fiber can function as prebiotics and support the activity of selected gut microorganisms.
Is yogurt a probiotic?
Some yogurts contain live cultures, but live microorganisms alone are not enough for every yogurt to be considered probiotic in the strict scientific sense. A probiotic benefit needs to be associated with adequately defined microorganisms provided in an appropriate amount.
Do all fermented foods contain probiotics?
No, because some fermented foods no longer contain live microorganisms after processing. In other products, the microorganisms may remain alive but may not have a specifically demonstrated probiotic benefit.
Does a higher CFU count mean a better probiotic?
Not necessarily, because the number of viable microorganisms is only one part of the product. The strain, microorganism, dose, and evidence for the intended use also need to be considered.
Do probiotics need to be refrigerated?
It depends on the product because some probiotics require refrigeration while others are formulated to remain stable at room temperature. The manufacturer’s storage instructions should therefore be followed.
Final thoughts
Probiotics are live microorganisms that, when provided in adequate amounts, may offer health benefits. They are not one single product or one single type of microorganism.
Species, strain, and amount all matter, and effects demonstrated for one probiotic should not automatically be attributed to every probiotic product. This is one of the most important distinctions to remember when reading claims about probiotic benefits.
Fermented foods and probiotic supplements are also different categories. Some fermented foods may contain live microorganisms, but that alone is not enough to classify them automatically as probiotics.
When choosing a supplement, CFU is only one part of the label. Microorganism identity, strain, Serving Size, storage conditions, and overall product quality should all be considered together.
References
- National Center for Complementary and Integrative Health. Probiotics: Usefulness and Safety. nccih.nih.gov
- National Center for Complementary and Integrative Health. 5 Things To Know About Probiotics. nccih.nih.gov
- NIH Office of Dietary Supplements. Dietary Supplements for Immune Function and Infectious Diseases — Probiotics. ods.od.nih.gov
- Hill C, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology & Hepatology. 2014. nature.com
- Marco ML, et al. The International Scientific Association for Probiotics and Prebiotics consensus statement on fermented foods. Nature Reviews Gastroenterology & Hepatology. 2021. nature.com
- Salminen S, et al. The International Scientific Association of Probiotics and Prebiotics consensus statement on the definition and scope of postbiotics. Nature Reviews Gastroenterology & Hepatology. 2021. nature.com
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