At the beginning of the 20th century, Russian scientist Elie Metchnikoff determined that a diet of fermented milk products resulted in a long, healthy life for Bulgarian peasants [source: Sanders]. Probiotics, meaning "for life" in Greek, became the term for the bacteria that was found in those fermented products. The bacteria have a very technical definition provided by the World Health Organization; they are "live microorganisms which, when administered in adequate amounts, confer a health benefit on the host" [source: Brody].
The two main types of bacteria considered to be probiotics include strains from the Lactobacillus and Bifidobacterium genera (other genera, such as Escherichia, Enterococcus and Saccharomyces have also been designated probiotics, but to a lesser and more questionable extent). Lactobacillus and Bifidobacterium are made up of different strains, each of which has a different health function. To truly understand the potential benefit to your body, you have to research the difference between Lactobacillus acidophilus and Lactobacillus rhamnosus.
In general, though, scientists believe that probiotics work by repopulating the gut with good bacteria, which can be eliminated along with bad bacteria during a course of antibiotics. Additionally, probiotics are believed to have the power to fight off pathogens and toxins, as well as strengthen the gut fortresses that will eventually have to do battle with the same.
How does this hypothesis of how probiotics work translate into a health benefit? In some cases, the evidence is a bit shaky. Right now, the clearest benefit of probiotics, backed up by scientific study, comes in the field of gastrointestinal conditions, such as antibiotic-associated diarrhea, acute infectious diarrhea (such as traveler's diarrhea) and irritable bowel syndrome. A meta-analysis of 19 studies showed that probiotics reduced the risk of developing diarrhea associated with antibiotics by 52 percent, and cut the risk and duration of infectious diarrhea as well. Probiotics are successful in treating diarrhea caused by rotavirus in children and ulcerative colitis. However, probiotics weren't shown to be effective for acute pancreatitis and Crohn's disease, according to the American Family Physician in a 2017 study.
Another study, published in 2018 in the journal Cell, suggested some people may be resistant to supplemented probiotic bacteria, and therefore get no benefit from it at all. The researchers also investigated whether probiotics can help the gut microbiome bounce back after a round of antibiotics, and they found that, though probiotics might have helped with diarrhea related to the antibiotic, they seem to have delayed the reconstitution of gut bacteria.