Probiotics and Prebiotics: Definitions, Types, and Benefits

The words sound alike and are constantly mixed up, but probiotics and prebiotics are completely different things. Probiotics are live microbes you swallow to add helpful bacteria to your gut. Prebiotics are not alive at all: they are special fibers that feed the helpful bacteria already living there. One adds the workers; the other feeds them. Get this distinction clear and the rest of the topic, including synbiotics and postbiotics, falls into place.

What are probiotics?

Probiotics are defined as live microorganisms that, when given in adequate amounts, confer a health benefit on the host. This definition, from the World Health Organization and the International Scientific Association for Probiotics and Prebiotics (ISAPP), has three parts that all matter: the microbes must be alive, given in enough quantity, and shown to do something beneficial.

Two consequences follow directly from that definition. First, dead microbes and substances made by microbes are not probiotics (they have their own name, postbiotics, discussed below). Second, a probiotic must have demonstrated benefit, so not every microbe in a fermented food or every product labeled "probiotic" qualifies.

Probiotics are mostly bacteria, but some are yeasts (notably Saccharomyces boulardii). They are taken to restore or support a healthy gut community, which can be disturbed by illness, antibiotics, surgery, or poor diet. They are available as foods (such as yogurt and kefir), dietary supplements, and in some cases drugs.

Probiotics, prebiotics, synbiotics, and postbiotics

Four related terms are constantly confused. Keeping them straight is the single most useful thing to learn in this topic.

The key contrasts: probiotics are alive, prebiotics are not. Prebiotics feed the good bacteria already in your gut rather than adding new ones. Synbiotics pair the two. Postbiotics are not live microbes at all, which is why they fall outside the strict definition of a probiotic.

For a full treatment of prebiotics (their types, sources, and how they work), see the dedicated article on prebiotics. For how probiotics actually produce their effects in the gut, see the article on how probiotics work.

Prebiotics

Prebiotics are non-digestible food ingredients, typically complex carbohydrates such as oligosaccharides and other fibers, that escape digestion in the upper gut and reach the colon, where they selectively feed beneficial bacteria like bifidobacteria and lactobacilli. Common examples include oligofructose, inulin, galacto-oligosaccharides, lactulose, and the oligosaccharides in human breast milk. They occur naturally in foods such as onions, garlic, leeks, bananas, chicory root, and asparagus. By feeding beneficial gut bacteria, prebiotics can increase bifidobacteria, improve calcium absorption, add fecal bulk, and shorten gut transit time.

Figure: Prebiotics, Probiotics and Synbiotics

The full detail on prebiotic types, food sources, and mechanisms is covered in the dedicated article on prebiotics. When a product combines a probiotic and a prebiotic together, it is called a synbiotic, giving both effects at once.

Figure: Probiotic vs antibiotics

Probiotics vs. Antibiotics

The names are opposites and so are the actions. "Probiotic" means "for life"; "antibiotic" means "against life." Antibiotics kill bacteria to treat infection, but they cannot tell harmful bacteria from the helpful gut bacteria, so a course of antibiotics often depletes the good bacteria too. This is why antibiotic use can cause diarrhea, and why probiotics are sometimes used alongside or after antibiotics to help restore the gut community.

Figure: Action of antibiotics and probiotics

Microorganisms used as probiotics

Seven groups of microorganisms account for most probiotic products: Lactobacillus (and the genera it was recently split into), Bifidobacterium, Saccharomyces, Streptococcus, Enterococcus, Escherichia, and Bacillus. Most are bacteria; the main exception is the yeast Saccharomyces boulardii. To be effective, a probiotic product generally needs a high dose of live cells (commonly 10⁶ to 10⁹ or more per serving).

A note on names: in 2020 the large genus Lactobacillus was reclassified into many new genera (such as Lactiplantibacillus, Limosilactobacillus, and Lacticaseibacillus). Older product labels and papers still use the familiar Lactobacillus names, so both appear in practice. This article uses the widely recognized names while noting the change.

A brief look at the main groups:

• Lactobacillus and related genera: acid- and bile-tolerant rods, normally found in the gut and vagina. Common probiotic species include L. acidophilus, L. rhamnosus GG, L. casei, L. plantarum, and L. reuteri. Used for antibiotic-associated diarrhea, some infections, and gut health.
• Bifidobacterium: anaerobic residents of the infant and adult gut. Species like B. bifidum, B. longum, B. breve, and B. lactis are used for constipation, diarrhea, and immune support.
• Saccharomyces boulardii: a probiotic yeast, used especially for antibiotic-associated and infectious diarrhea. Because it is a yeast, it is unaffected by antibacterial antibiotics.
• Streptococcus thermophilus: a yogurt organism that, with Lactobacillus delbrueckii subsp. bulgaricus, improves lactose digestion.
• Enterococcus (E. faecium): used in some products, but with caution, as certain strains can act as opportunistic pathogens; now mostly limited to animal use.
• Escherichia coli Nissle 1917: a specific well-studied strain used for some inflammatory bowel conditions.
• Bacillus (B. coagulans): spore-forming, so it survives storage and stomach acid well.

Why strain matters: the single most important idea

Probiotic effects are strain-specific. A benefit shown for one strain does not transfer to another strain, even of the same species. This is why probiotics are named down to the strain, using genus, species, and an alphanumeric strain code, for example Lactobacillus rhamnosus GG or *Bifidobacterium* infantis 35624. When choosing or studying a probiotic, the strain designation, not just the species, is what tells you what evidence applies.

Figure: Beneficial effects of common probiotic Bifidobacterium strains.

A microorganism is suitable as a probiotic only if it survives the journey and does its job. The desirable properties are: resistance to stomach acid and bile, ability to adhere to and transiently colonize the gut lining, antimicrobial activity against pathogens, a demonstrated beneficial effect, and safety (nontoxic, nonpathogenic).

How to choose a probiotic

There are no official recommendations for probiotic use in healthy people, and the best advice is to ask a healthcare provider which strain, dose, and duration suit your specific need. A few evidence-based principles help when reading labels:

• Match the strain to the need. Because effects are strain-specific, look for the exact strain (not just the species) that has been studied for your particular concern, such as antibiotic-associated diarrhea or IBS.
• Check the dose. The label should state the number of live organisms (CFU) and, ideally, that this count is guaranteed through the expiration date, not just at the time of manufacture.
• Confirm viability and storage. Probiotics are live cultures. Check the "use by" date and follow storage instructions; some products need refrigeration, others are shelf-stable.
• Use caution if immunocompromised. Probiotics are generally safe for healthy people, but in people with weakened immune systems they can, rarely, cause infections such as bacteremia or fungemia. Medical advice is important in that situation.

The number of commercial brands is large and changes constantly. Rather than choosing by brand, choose by the strain and the evidence behind it, then confirm the product actually contains that strain at an adequate, guaranteed dose.

Health benefits of probiotics

The best-documented benefit of probiotics is in preventing and treating certain forms of diarrhea, particularly antibiotic-associated diarrhea and some infectious diarrheas. This is where the evidence is strongest.

For other conditions, the evidence is promising but less settled. Probiotics may help with some symptoms of irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), and may improve lactose digestion in people who are lactose intolerant, but effects depend heavily on the specific strain, and large trials are still needed before doctors can routinely prescribe particular probiotics for these conditions.

Figure: Beneficial roles of Lactic acid bacteria

Benefits that have been reported, with varying strength of evidence, include:

- Reducing the risk of antibiotic-associated diarrhea

- Helping control intestinal pathogens

- Improving lactose digestion in lactose-intolerant people

- Possible modest reductions in serum cholesterol

- Possible benefits in some IBS and IBD symptoms

Because benefits are strain-specific and the field is still developing, probiotics are best seen as a supportive measure for specific, evidence-backed uses rather than a general remedy. Anyone considering probiotics for a medical condition should speak with a clinician.

How to Remember

Probiotic vs prebiotic: live workers vs their food.
Probiotics are live microbes you add. Prebiotics are the fiber that feeds the microbes already there. "Pro" = the live bugs; "pre" = what comes before them as food. If it's alive, it's a probiotic; if it feeds them, it's a prebiotic.

The four "-biotics" in one line.
Probiotic (live microbes), prebiotic (their food), synbiotic (both together), postbiotic (what microbes leave behind, or dead microbes). Read them as a sequence: feed (pre), add (pro), combine (syn), leftovers (post).

Strain is everything.
A benefit proven for L. rhamnosus GG does not apply to a different L. rhamnosus strain. The alphanumeric code (GG, 35624, Nissle 1917) is the part that carries the evidence. Species alone is not enough.

The one benefit with the strongest evidence: diarrhea.
Of all probiotic claims, prevention and treatment of certain diarrheas (especially antibiotic-associated) is the best supported. Start there when you think about what probiotics reliably do.

Key exam facts in one table

Where Students Get Confused

What is the difference between probiotics and prebiotics?
Probiotics are live beneficial microbes you consume. Prebiotics are non-living fibers that feed the beneficial microbes already in your gut. One adds the bacteria; the other feeds them. They are often taken together as a synbiotic.

Are all fermented foods probiotic?
No. A fermented food is only probiotic if it contains live microbes in adequate amounts that have a demonstrated benefit. Many fermented foods are pasteurized or processed after fermentation, which kills the microbes, so they no longer qualify as probiotic even though they were made by fermentation.

Why does the strain matter so much?
Because probiotic effects are strain-specific. Two strains of the same species can have completely different effects. Evidence gathered for one strain does not automatically apply to another, which is why the alphanumeric strain code (like GG or Nissle 1917) matters more than the species name.

Are probiotics and postbiotics the same?
No. Probiotics are live microbes. Postbiotics are the beneficial substances microbes produce, or the dead microbes themselves. Because postbiotics are not alive, they fall outside the strict definition of a probiotic.

Are probiotics always safe?
For most healthy people, yes. But in people with weakened immune systems, probiotics can rarely cause serious infections such as bacteremia or fungemia. Anyone who is immunocompromised or seriously ill should check with a doctor before taking them.

References and further readings

1. Hill C, Guarner F, Reid G, et al. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nat Rev Gastroenterol Hepatol. 2014;11(8):506–514. https://doi.org/10.1038/nrgastro.2014.66
2. Fijan S. Microorganisms with claimed probiotic properties: an overview of recent literature. Int J Environ Res Public Health. 2014;11(5):4745–4767. https://doi.org/10.3390/ijerph110504745
3. Hemarajata P, Versalovic J. Effects of probiotics on gut microbiota: mechanisms of intestinal immunomodulation and neuromodulation. Therap Adv Gastroenterol. 2013;6(1):39–51. https://doi.org/10.1177/1756283X12459294
4. Zheng J, Wittouck S, Salvetti E, et al. A taxonomic note on the genus Lactobacillus: description of 23 novel genera. Int J Syst Evol Microbiol. 2020;70(4):2782–2858. https://doi.org/10.1099/ijsem.0.004107
5. Office of Dietary Supplements. Probiotics: Health Professional Fact Sheet. National Institutes of Health. Available from: https://ods.od.nih.gov/factsheets/Probiotics-HealthProfessional/
6. World Gastroenterology Organisation. Probiotics and Prebiotics. WGO Global Guidelines; 2017.