Overview

Per the WHO/FAO 2001 definition (still the field’s reference standard), probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. Importantly, not every fermented food qualifies as “probiotic” in the strict sense — a food only counts if it contains defined, viable strains in sufficient numbers, which is why some fermented products (e.g., beer, wine, most bread, and pasteurized or vinegar-based pickles) don’t count even though fermentation was part of their production.

Key strains and food sources

Yogurt and fermented dairy: traditional yogurt cultures are Lactobacillus bulgaricus and Streptococcus thermophilus; many commercial yogurts add further strains such as L. acidophilus or Bifidobacterium. Kefir typically contains a much broader and more diverse set of bacterial and yeast strains than yogurt. Look for “live and active cultures” on the label.
Fermented vegetables: traditional sauerkraut (dominated by L. plantarum) and kimchi are lacto-fermented in a salt brine, not pasteurized — refrigerated, raw versions in the grocery store retain live cultures; shelf-stable, vinegar-brined, or pasteurized versions generally do not.
Saccharomyces boulardii, a probiotic yeast, has clinical trial support specifically for reducing antibiotic-associated diarrhea.

CFU counts (colony-forming units) on labels typically range from roughly 1 to 50 billion per serving for common commercial products. Viability matters more at the point of consumption/expiration than at time of manufacture, since live cultures can decline in number over shelf life, especially without refrigeration.

Gut-brain axis and immune effects

The gut is connected to the brain via the vagus nerve and produces or influences several neurotransmitters (a large share of the body’s serotonin is produced in the gut, for example). This has generated substantial interest in “psychobiotics” — probiotic strains studied for mood and stress effects — but this is still an emerging research area; results vary by strain and are not yet strong enough to support specific clinical claims for anxiety or depression treatment.

A large proportion of the body’s immune tissue is associated with the gut lining, and probiotics are thought to support barrier function and compete with pathogenic bacteria for space and nutrients, though effects are strain-specific rather than a general “probiotics are good” statement.

Safety and contraindications

Fermented probiotic foods are safe for most healthy adults. Immunocompromised individuals, and people with central venous catheters or serious underlying illness, should discuss probiotic supplements (higher-dose than food) with their physician, as rare cases of bacteremia/fungemia have been reported in high-risk patients.

Sources

Clinically reviewed by Dr. Shilpa Thakur. Last reviewed 2026-07-20. Educational information, not medical advice; consult a qualified clinician before starting probiotic supplements if immunocompromised or seriously ill.