What Is a Probiotic?

The concept of beneficial microbes dates to Nobel laureate Elie Metchnikoff in 1907, who proposed that yoghurt-eating Bulgarian peasants lived longer because of lactic acid bacteria. Probiotics, from Greek/Latin meaning "for life," include strains of Lactobacillus, Bifidobacterium, and the yeast Saccharomyces boulardii. They work through several mechanisms: competing with pathogenic bacteria for nutrients and attachment sites, producing antimicrobial compounds, strengthening the gut barrier, and modulating the immune system. Clinical evidence supports their use for antibiotic-associated diarrhoea, irritable bowel syndrome, and certain allergic conditions. The human gut contains about 38 trillion microorganisms (roughly equal to the number of human cells), and this microbiome influences digestion, immunity, mood, and even body weight. Fermented foods like yoghurt, kefir, kimchi, and sauerkraut are natural probiotic sources.