probiotic
Meaning of probiotic: Strain is everything here, since evidence gathered for one probiotic organism does not carry over to a different organism in a different product.
Definition of probiotic
A probiotic is a live microorganism, usually a bacterium and sometimes a yeast, taken in an amount intended to produce a health benefit. Probiotics come as capsules, powders and drinks, and occur naturally in fermented foods such as yogurt, kefir and sauerkraut. What a product does depends on the exact species and strain it contains, not on the general idea of adding bacteria.
Which organisms are used?
Most products draw on a short list of genera, and each named product specifies a strain within a species, written as a code after the species name. That code is not decoration. Two strains of the same species can differ in whether they survive stomach acid, stick to the gut lining, or do anything measurable at all.
| Organism | What it is | Typical source |
|---|---|---|
| Lactobacillus and related genera | Acid-tolerant bacteria, the most widely used group | Yogurt, fermented milk, supplements |
| Bifidobacterium | Bacteria abundant in the infant large intestine | Supplements, some yogurts |
| Saccharomyces boulardii | A yeast rather than a bacterium, so antibiotics do not kill it | Supplements |
| Mixed preparations | Several species combined in one product | Supplements, fermented drinks |
How do they relate to the gut microbiome?
The large intestine already holds an enormous resident community of bacteria, fungi and viruses built up over a lifetime. A swallowed microorganism is a visitor in that setting. Most probiotic strains pass through and disappear from the stool within days of the last dose, so any effect depends on continued intake rather than on permanent colonization. That is why probiotics are described as supporting the existing community rather than replacing it. A related term, prebiotic, means a food component such as inulin that feeds resident bacteria instead of adding new ones.
What is actually known about the effects?
Evidence is uneven and organism-specific. It is strongest for a few narrow situations, including antibiotic-associated diarrhea and some infectious diarrhea in children, where particular strains such as Lactobacillus rhamnosus GG and Saccharomyces boulardii have been studied repeatedly. Claims spread far wider than that, into mood, immunity, weight and skin, on much thinner ground. Regulation adds to the confusion, because in many countries probiotics are sold as foods or supplements and are not reviewed for effectiveness before sale the way a medication is. Products also vary in whether the organisms are still alive at the point of use.
Used in a sentence
The patient asked whether a probiotic supplement might help with ongoing digestive symptoms.
Good to know
The effects of a probiotic are specific to the particular species and strain used, so evidence supporting one probiotic product does not necessarily apply to a different product containing a different organism.
Word origin
Where “probiotic” comes from
Formed from pro-, from Greek meaning for or on behalf of, combined with bios, meaning life, contrasting with antibiotic, formed using the opposite prefix anti-, meaning against.