IngredientsThe International Scientific Association for Probiotics and Prebiotics defines probiotics as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host." These microorganisms consist mainly of bacteria but also include yeasts.
Probiotic bacteria are classified within the domain Bacteria, and the vast majority of supplement-grade organisms belong to two principal genera:
The most predominant genera used in probiotic supplements are Lactococcus, Lactobacillus, and Bifidobacterium spp. Other bacterial organisms used in supplemental and fermented food contexts include species from the genera Streptococcus (e.g., S. thermophilus), Enterococcus, Bacillus, Propionibacterium, and Escherichia coli strain Nissle 1917. Other bacteria may also be used as probiotics, and so may yeasts such as Saccharomyces boulardii.
Probiotics are living microorganisms that confer health benefits to the host when administered in adequate amounts; however, dead bacteria and their components can also exhibit probiotic properties. Bifidobacterium and strains of lactic acid bacteria are the most widely used bacteria that exhibit probiotic properties and are included in many functional foods and dietary supplements.
At the taxonomic level, strain identity is of clinical importance. Different types of probiotics may have different effects. For example, if a specific kind of Lactobacillus helps prevent an illness, that doesn't necessarily mean that another kind of Lactobacillus or any of the Bifidobacterium probiotics would do the same thing.
Probiotic microorganisms are present in some fermented foods (e.g., yogurt), are added to certain food products, and are available as dietary supplements. Key food-based natural sources include:
Probiotics are available to consumers in oral products such as dietary supplements and yogurts, as well as other products such as suppositories and creams. Dietary supplement formats include capsules, tablets, powders, liquids, gummies, and sachets. Because probiotics must be consumed alive to have health benefits and they can die during their shelf life, users should look for products labeled with the number of CFU at the end of the product's shelf life, not at the time of manufacture.
Probiotics are considered dietary supplements; thus, they are not covered by medical insurance and their production is not regulated by the Food and Drug Administration. As such, product quality, purity and viability have been reported to be variable. However, several clinically tested probiotic products with quality-controlled production are now marketed by reputable companies.
Although the concept of bacteria was not yet understood, many cultures throughout history consumed fermented foods that we now know contained probiotics. These foods were usually developed in an effort to preserve food for longer periods of time, but people soon noticed that eating them also improved digestion and overall health. Through trial and error, ancient people discovered that fermented foods provided numerous health benefits and passed down their knowledge through generations. However, fermented foods weren't just used for preservation during the history of probiotics — they were also believed to have medicinal properties.
The consumption of fermented foods for their beneficial health effects is an old practice; yoghurt, cheese, and other fermented foods were widely consumed in ancient Egypt, China, India, and Africa for their health benefits.
In ancient Greece and Rome, fermented milk was used to treat digestive and intestinal disorders, as well as wounds and infections. The history of probiotics can be traced to the first use of cheese and fermented products, which were well-known to the Greeks and Romans who recommended their consumption. The fermentation of dairy foods represents one of the oldest techniques for food preservation.
In traditional medicine systems such as Ayurveda and traditional Chinese medicine, the importance of gut health was recognized, and the consumption of fermented foods was recommended to promote a healthy digestive system.
Early scientific insights into probiotics followed Louis Pasteur's discovery of the role of microbes in fermentation. In 1907, Elie Metchnikoff, a Russian Nobel Prize-winning scientist, linked improved health and longevity among Bulgarian peasants to the consumption of fermented milk containing Lactobacillus bulgaricus. While Pasteur discovered the microorganisms responsible for fermenting food, Metchnikoff was the first to theorize that these and other microorganisms themselves benefit human health.
Lactic acid bacteria with health-promoting qualities were first isolated in the 1920s. In the 1930s, Minoru Shirota in Japan isolated Lactobacillus casei Shirota and, in 1935, created Yakult, a probiotic drink.
Bulgarian bacillus, later named Lactobacillus bulgaricus, was shown to be unable to survive in the human digestive system by Leo F. Rettger of Yale in 1921. This discovery caused a fall-off of the fermented food phenomena. Rettger continued to investigate different strains of Lactobacillus, however, concluding in 1935 that certain strains of Lactobacillus acidophilus not only could survive the environment of the human gut — they were very active.
Between the 1970s and 1990s, the use of probiotics for gut health increased in Asia and Europe. Probiotics have received renewed attention in the 21st century from product manufacturers, research studies, and consumers.
The community of microorganisms that lives on us and in us is called the "microbiome," and it's a hot topic for research. The Human Microbiome Project, supported by the National Institutes of Health (NIH) from 2007 to 2016, played a key role in this research by mapping the normal bacteria that live in and on the healthy human body.
Unlike botanical or chemical supplements, the primary "active ingredient" in bacterial probiotics is the intact, living microorganism. Strain identity — defined to genus, species, and strain level — determines specific probiotic properties. However, certain bacterial metabolites and structural components are recognized as mediators of biological effects.
SCFAs, mainly butyrate, acetate and propionate, play several key roles in human health, from the modulation of the immune system to the regulation of metabolic pathways and the restoration of the gut barrier. Several bacteria are able to degrade substrate sources (mainly non-digestible polysaccharides, but also, less frequently, proteins) to produce SCFAs.
Short chain fatty acids (SCFAs), as a bacterial fermentation terminal product with multiple metabolic features, are crucial energy sources for the intestinal microbiota and host intestinal epithelium cells (IEC), which could maintain intestinal acid-base balance, inhibit the growth of harmful pathogens, modulate the host intestinal immunity, and thus reduce inflammatory responses.
At the molecular level, SCFAs, mainly butyrate, increase the concentration of tight junctions, such as claudin-1, zonula occludens-1 and occludin through the upregulation of genes that encode for these proteins. Moreover, butyrate is able to strengthen the mucus layer of the gut epithelium by increasing the expression of Mucin 2. Butyrate is also involved in the modulation of oxidative stress, as it reduces H2O2-induced DNA damage, restoring the levels of antioxidant glutathione.
The inhibition of E. coli O157:H7 by different Lactococcus and Lactobacillus strains was attributed to the production of lactic acid and low pH. The growth of Helicobacter pylori was inhibited by different Lactobacillus and Bifidobacterium strains including L. acidophilus, L. bulgaricus and Bifidobacterium bifidus. These effects were linked to the production of lactic, acetic and hydrochloric acid.
Recent work has shed light on Bifidobacterium surface structural polysaccharide and protein elements, as well as its metabolic products, as commensal mediators of immune homeostasis. Lactic acid bacteria are indigenous to the small and large intestine of humans and animals and exert a number of probiotic properties, such as binding to receptors and physically excluding pathogens, production of antimicrobial substances, strengthening of the gut mucosal barrier and modulation of the immune system.
Probiotic microorganisms influence brain function through defined mechanisms, including modulation of neuroinflammation, neurotransmitter production (GABA, serotonin), regulation of the hypothalamic–pituitary–adrenal (HPA) axis, and vagus nerve signaling.
The ability of probiotics to modify the immunological response of the host, antagonize pathogenic microbes, or compete for adhesion sites with pathogenic microorganisms is related to the action of probiotics against microorganisms. The growth of four species of known enteropathogens — H. pylori, Campylobacter jejuni, Campylobacter coli, and C. difficile — was inhibited by Lactobacillus strains isolated from the human GIT, probably due to the production of organic acids. Based on these studies, it is reasonable to suggest that the production of organic acids by probiotics in the GIT makes the intestinal environment less favorable for their competitors and decreases the risk of enteric infections by pathogens.
SCFAs not only serve in the intestine where commensal bacteria reside, but also have a critical role in improving the barrier capacity of the organism's intestine and resisting invasion by foreign pathogens. Probiotics can restore microbial balance and thus inhibit the proliferation of pathogens such as C. difficile.
At the cellular level, Bifidobacterium upregulates suppressive regulatory T cells, maintains intestinal barrier function, modulates dendritic cell and macrophage activity, and dampens intestinal Th2 and Th17 programs. Depletion or absence of Bifidobacterium in humans and model organisms is associated with autoimmune responses and impaired immune homeostasis.
The mechanism of probiotic treatment for IBD may be related to reducing oxidative stress, repairing the intestinal barrier, regulating intestinal flora balance, and modulating intestinal immune response.
The gut microbiome plays a significant role in regulating gastrointestinal (GI) function and modulating the gut–brain axis, which describes the bidirectional communication between the GI tract and the central nervous system (CNS). Recent studies have revealed that the gut microbiota is involved in communication with the brain, through a bidirectional communication network known as the gut-brain axis. This communication involves humoral, immunological, endocrine, and neural pathways. Gut dysbiosis negatively impacts these communication pathways, leading to neurological complications and cognitive deficits.
Clinical, epidemiological, and immunological evidence suggest that enteric microbiota extensively and profoundly influences the gut-brain relationship — i.e., mental state, emotional regulation, neuromuscular function, and regulation of the HPA.
Probiotics have been shown to prevent and ameliorate the course of digestive disorders such as acute, nosocomial, and antibiotic-associated diarrhea; allergic disorders such as atopic dermatitis (eczema) and allergic rhinitis in infants; and Clostridium difficile–associated diarrhea and some inflammatory bowel disorders in adults.
Statements with 100% agreement and 'high' evidence levels indicated that: (i) specific probiotics help reduce overall symptom burden and abdominal pain in some IBS patients; (ii) in patients receiving antibiotics/Helicobacter pylori eradication therapy, specified probiotics are helpful as adjuvants to prevent/reduce the duration/intensity of AAD; (iii) probiotics have favourable safety in patients in primary care.
Clinical trials have assessed the therapeutic effects of probiotics for several disorders, including antibiotic- or Clostridium difficile-associated diarrhea, irritable bowel syndrome, and the inflammatory bowel diseases. Although probiotic research is a rapidly evolving field, there are sufficient data to justify a trial of probiotics for treatment or prevention of some of these conditions. However, the capacity of probiotics to modify disease symptoms is likely to be modest and varies among probiotic strains — not all probiotics are right for all diseases.
Evidence strength: The evidence for probiotics in preventing or shortening AAD is among the strongest in the field, with multiple RCTs and systematic reviews, including Cochrane reviews, supporting specific strains such as L. rhamnosus GG and Saccharomyces boulardii. The benefit is clinically meaningful but modest, and is strain-specific.
Many pieces of evidence have suggested that probiotics may benefit IBS symptoms. However, the results of clinical trials have been conflicting.
In a meta-analysis incorporating 72 randomized controlled trials (RCTs), probiotics showed a medium effect size on the improvement of global IBS symptoms (standardized mean difference, −0.55, 95% CI −0.76 to −0.34) compared with placebo.
Thirty-seven studies were included in a systematic review; mostly on irritable bowel syndrome (IBS; 19 studies; treatment responder rates: 18–80% (specific probiotics), 5–50% (placebo)) or antibiotic-associated diarrhoea (AAD; 10 studies). Items with 70–100% agreement and 'moderate' evidence were: (i) specific probiotics help relieve overall symptom burden in some patients with diarrhoea-predominant IBS, and reduce bloating/distension and improve bowel movement frequency/consistency in some IBS patients; and (ii) with some probiotics, improved symptoms have led to improvement in quality of life.
A meta-analysis indicated that probiotics may be beneficial and safe to improve symptoms of IBS compared with placebo. However, it was difficult to draw a precise conclusion as a result of the existence of significant heterogeneity and possible publication bias.
For clinicians, routine prescription of probiotics as first-line treatment for IBS should be avoided. Their use may be considered as an individualized therapeutic trial in refractory patients, with discontinuation if there is no subjective response after 8 weeks, and without assuming equivalence between different probiotic strains or formulations.
Evidence strength: Moderate but heterogeneous. The overall effect size across pooled RCTs is statistically significant but the clinical significance varies considerably by strain, formulation, and IBS subtype. Although numerous systematic reviews have evaluated the effects of probiotics in IBS, uncertainty persists regarding their clinical effectiveness, methodological quality, and certainty of evidence.
Probiotics had a significant effect in inducing clinical remission in ulcerative colitis (UC). A subgroup analysis suggested that combining 5-ASA and probiotics may provide additional benefit. Evidence from the largest meta-analysis of probiotics in IBD, which incorporated 67 studies (22 systematic reviews and 45 RCTs), supports a role for probiotics — particularly multi-strain preparations — in UC.
Differences in the benefits of probiotics between Crohn's disease (CD) and UC may be attributable to the different lesion extent and immune-mediated pathophysiology. More robust randomized clinical trials are required to validate the efficacy and safety of diverse probiotic strains in IBD.
Several limitations exist with trials which have evaluated probiotic therapy in the inflammatory bowel diseases. These include small cohort sizes, use of different probiotic doses and strains, varied treatment durations and differences in concurrent conventional treatment.
Evidence strength: Moderate for UC (particularly for maintaining remission); weak to insufficient for CD. The evidence base is constrained by trial heterogeneity, small sample sizes, and the use of highly variable probiotic preparations.
Studies have evaluated the use of various probiotic products to prevent atopic dermatitis, a common inflammatory skin disorder. The results of these studies suggest that probiotics may reduce the risk of developing atopic dermatitis and the severity of symptoms, though the relief these products can provide may be limited.
Probiotics have been shown to prevent and ameliorate the course of allergic disorders such as atopic dermatitis (eczema) and allergic rhinitis in infants.
Evidence strength: Preliminary to moderate. Meta-analyses suggest some preventive effect, particularly when probiotics are administered perinatally and in infancy, but results are not consistent across all strains or populations. The ODS characterizes the relief as potentially limited.
A class of probiotics known as psychobiotics has emerged, with targeted neuroactive properties. These microorganisms influence brain function through defined mechanisms, including modulation of neuroinflammation, neurotransmitter production (GABA, serotonin), regulation of the hypothalamic–pituitary–adrenal (HPA) axis, and vagus nerve signaling.
The gut–brain axis is bidirectional, meaning that chronic stress and central nervous system abnormalities can alter the composition and function of the gut microbiota, creating a vicious cycle. In both kainic acid-induced epilepsy models and clinical trials involving patients with Alzheimer's disease and treatment-resistant depression, probiotic treatment resulted in notable decreases in systemic and central inflammation markers. These results indicate that immunomodulation is a core mechanism in the gut–brain interaction facilitated by probiotics.
A probiotic beverage containing L. acidophilus, L. fermentum, L. casei, and B. bifidum for 12 weeks significantly improved Mini-Mental State Examination score in 60 patients with Alzheimer's disease, with a mean age of 80.
Not all randomized clinical trials showed positive impacts of probiotics on subjects' cognitive health. The mechanisms underlying these effects and their long-term clinical relevance remain incompletely understood.
Evidence strength: Preliminary. Research is rapidly evolving, with some positive signals from clinical trials in depression, anxiety, and cognitive decline, but the field lacks large, well-controlled RCTs. Many findings derive from animal models or small human studies with heterogeneous designs.
In addition, probiotics may be of interest as coadjuvants in the treatment of metabolic disorders, including obesity, metabolic syndrome, nonalcoholic fatty liver disease, and type 2 diabetes.
The potential health benefits of probiotics are the focus of a great deal of scientific research. Key health conditions under investigation include hypercholesterolemia and obesity.
Evidence strength: Preliminary to moderate. Some RCTs report modest improvements in lipid profiles and insulin sensitivity with specific strains, but effect sizes are generally small and clinical significance is uncertain. These findings are not considered sufficient to support routine clinical use for these indications.
Lower levels of bifidobacteria, including the species B. adolescentis, have been identified in adult subjects with allergic asthma, suggesting a potential protective role of bifidobacteria in respiratory diseases. Additionally, it has been demonstrated that treatment with B. adolescentis reduces allergic inflammation in the airways of murine models, indicating a potential therapeutic effect for allergic asthma. However, the mechanisms by which these probiotic microorganisms exert these benefits are not fully understood.
Evidence strength: Largely preliminary; the bulk of data is from observational studies and animal models. Human clinical trial evidence is limited and inconsistent.
Lactobacilli protects women from developing urinary tract infections (UTIs). Studies have shown that the two-strain combination of distal urethral isolates L. rhamnosus GR-1 and L. fermentum B-54 with RC-14 works through anti-gram-positive bacilli activities and hydrogen peroxide production.
Evidence strength: Preliminary to moderate for vaginal health and UTI prevention in women; further high-quality RCTs are needed.
Infections of the digestive tract, irritable bowel, lactose intolerance, allergies, infections of the urogenital tract, cystic fibrosis, and various cancers can all be potentially prevented and treated with the use of probiotics. The body systems most consistently studied in connection with probiotic bacteria include:
The body, especially the lower gastrointestinal tract (the gut), contains a complex and diverse community of bacteria. Although we tend to think of bacteria as harmful "germs," many bacteria actually help the body function properly.
Probiotic dosage is expressed in colony-forming units (CFU), a measure of viable microorganism quantity. CFU estimates the number of live microbes capable of forming colonies in laboratory testing. Supplemental probiotics come in a wide range of CFUs — from the high millions to as many as 200 billion.
Although the vast majority of existing clinical trials indicate that probiotic doses of 10–20 billion CFU per day are sufficient for maintaining immune and digestive health, research studies examining the dose-response of larger CFUs and products featuring CFUs of 50 to 100 billion are becoming increasingly common.
It is worth noting that the number of CFUs in a probiotic can decline over time. Therefore, the number of CFUs that a product contained when it was manufactured does not necessarily reflect the number of CFUs that the product contains when it is purchased or used. For this reason, some experts recommend choosing products that list the number of CFUs that the product will contain by the expiration date or use-by date instead of the number it contained at the time of manufacture.
Specific dosages reported in clinical studies vary widely by strain and indication. Representative examples from the literature include:
There is no single universally established optimal dose. Optimal dosing is strain-specific, indication-specific, and can depend on formulation and viability at time of consumption. Different types of probiotics may have different effects, and this principle extends to dosage requirements.
Probiotics are used by millions of people for their documented or anticipated health benefits. They usually belong to the genera lactobacilli and bifidobacteria and are part of the normal flora. They have been studied in a wide variety of diseases and in the prevention of inflammatory and infectious diseases. In healthy adults and children, probiotics have a well-documented record of safety, with adverse events generally limited to mild and transient gastrointestinal complaints such as bloating and flatulence.
Lactobacillus rhamnosus GG (LGG) is the most studied probiotic with the most thorough information on safety aspects.
In 2023, the FDA warned health care providers that premature infants who are given probiotics are at risk of severe, potentially fatal infections caused by the microorganism.
The FDA has reported that probiotics might cause infections or even life-threatening illness in preterm infants. Probiotics might also cause problems such as bacterial infections in people who are already seriously ill or who have weak immune systems.
On rare occasions, probiotics may translocate from the gastrointestinal tract, resulting in invasive infection. A systematic review was conducted of sepsis, bacteremia, and fungemia associated with probiotic administration in children between 1995 and 2021. Of the 49 invasive infections reported, sepsis was most common. The majority of the children meeting the clinical definition of sepsis were under two years old and had a predisposing condition such as prematurity or an indwelling intravenous catheter, and 94% were treated successfully with antimicrobial therapy.
Evidence from short-term observations suggests that certain probiotic strains might behave as opportunistic pathogens in populations who are immunocompromised or stressed.
A few recent cases of bacteremia and/or sepsis associated with lactobacilli have been reported in patients with different underlying diseases such as ulcerative colitis in pediatric or adult patients, suggesting that an extensive damage of the colonic mucous membrane increases the risk of bacteremia.
Probiotic translocation may result in allergic reactions, harmful immunological consequences, opportunistic systemic and local infections, and the spread of antibiotic resistance. The WHO/FAO working group recommended that new probiotic strains be evaluated for safety by testing for antibiotic resistance, toxin production and hemolytic potential, assessing metabolic activities such as D-lactate production and bile salt deconjugation, conducting human studies to evaluate side effects and post-market surveillance of commercial consumers, and, ideally, studying their use in immunocompromised animals to determine infectivity of the probiotic organism.
Dietary supplement labels may make claims about how the product affects the structure or function of the body without FDA approval, but they aren't allowed to make health claims, such as saying the supplement lowers your risk of getting a disease, without the FDA's consent. If a probiotic is going to be marketed as a drug for treatment of a disease or disorder, it has to meet stricter requirements. It must be proven safe and effective for its intended use through clinical trials and be approved by the FDA before it can be sold.
Not all foods and dietary supplements that are labeled as probiotics have proven health benefits. Probiotic strain selection should focus on quality tested products with clinically demonstrated benefit for the given disorder.
Gene-based bacterial profiling studies from disease-affected humans have identified what may be novel "probiotics" such as Faecalibacterium prausnitzii and Clostridium species IV and XIVa. The identification, purification and repackaging of probiotic-derived soluble factors possessing proven capacity to modify biologic function may allow us to harness the power of probiotics while averting the potential risks associated with live bacteria. Some suggest that as these advances progress to the clinic we will shift from the term "probiotic" into the new world of "pharmabiotics."
The most clinically relevant known interaction involves antibiotics: antibiotic therapy can markedly reduce or eliminate probiotic bacterial viability in the gut. Common reasons for taking probiotic supplements include repopulating the gut with lactobacilli after taking antibiotics and aiding in the treatment of inflammatory bowel disease, lactose intolerance, and gastrointestinal infections. Spacing probiotic intake several hours from antibiotic administration is commonly recommended in clinical trials to minimize this interaction. Knowing the antibiotic sensitivity profile of a particular probiotic provides a strategy for eradication, if needed, and thus reduces the risk of long-term colonization in the unlikely case such risks should become apparent.
Some empirical concerns regarding the safety of probiotics are the occurrence of disease, adverse metabolic effects on the gastrointestinal tract and gene transfer events. Immunosuppressant medications used in transplant or autoimmune patients may compound risks, as the host immune system cannot adequately respond to potential translocation of probiotic organisms.
Health conditions that Bacteria may help support.
Body systems that Bacteria may help support.
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