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Definition: Probiotics from Black's Medical Dictionary, 43rd Edition

Living BACTERIA, which, when given in sufficient dose to colonise the intestine, may produce a beneficial effect for the host. Many probiotics are LACTIC ACID bacteria – for example, LACTOBACILLUS or bifidobacterium. They may help some cases of travellers’ DIARRHOEA, reducing the period of acute symptoms – particularly if the infection is caused by one of the ROTAVIRUSES and may be of use in eczema in young children. Many other claims are made for their benefit but these are not backed up by research evidence.

Summary Article: Probiotics
from Nutrition: Science, Issues, and Applications

Probiotics are live microorganisms that increase the number of “good” or health-promoting bacteria in the body. Helpful bacteria appear to heighten the body's resistance to microbes that cause various problems within the digestive system and in other physiological systems. Probiotic foods and supplements also could be useful in the treatment of digestive disorders, especially disorders involving colon function. Probiotics can be taken in the form of supplements or as food, such as yogurt and fermented foods which contain these live bacteria cultures. Lactobacillus and Bifidobacterium are two common strains of bacteria that humans consume to improve their bodies’ balance of microorganisms.

Scientists are just beginning to understand the many activities of the microorganisms that inhabit the human body and the effects of these activities on health and disease. This collection of microorganisms is collectively referred to as the “microbiome.” “Normal flora” is the term used to describe the harmless and helpful microorganisms, including bacteria, found in the gastrointestinal (GI) tract. They contribute to many biological processes, such as digestion. Normal flora not only aid digestion but also work to produce and release helpful compounds—such as vitamin K—and fight against the invasion and growth of harmful bacteria. More than 1,000 species of bacteria have been found to reside in the human GI tract, although most people have fewer than 500 species. Maintaining optimal GI health involves having the right number of bacteria and a healthful mix of health-promoting bacteria in the gut.

Probiotic products claim to promote various health benefits by contributing to a better microbiome in the GI tract. Evidence is strongest for the reduction of infectious diarrhea and antibiotic-associated diarrhea. Several studies have found that a variety of probiotic supplements help to reduce the severity of infectious diarrhea, especially in infants and children. Supplements containing Lactobacillus GG have been most widely tested, although many other bacterial strains also have been found to be helpful. One of the most dangerous side effects of antibiotic administration is overgrowth of the bacterium Clostridium difficile, or C. diff a normal resident of the GI tract in healthy people, but during antibiotic therapy—which kills off most GI bacteria as well as killing the target organism—C. diff can thrive and cause severe diarrhea, a condition particularly dangerous in sick people who are receiving antibiotic therapy. Probiotic therapies might reduce the likelihood of C. diff overgrowth during antibiotic treatments, although not all studies support this idea.

Probiotics have shown some promise for the treatment of irritable bowel syndrome (IBS). Irritable bowel syndrome symptoms, such as abdominal cramping, constipation, and diarrhea are exacerbated by feelings of stress. One study found that probiotics might lessen the effect of stress on the GI tract in laboratory mice (Sun et al., 2013). In this study, the bacteria introduced in the probiotic treatment appeared to inhibit the inflammation that worsens GI symptoms. Results for probiotic treatment of IBS in people have been mixed, with most health providers suggesting that patients try probiotics, but discontinue use if symptoms worsen. The probiotics used most frequently for people with IBS contain Lactobacillus or Bifidobacterium strains. Because of their potential ability to modulate inflammation and other immune-system activity, researchers are exploring the use of probiotics for the treatment of inflammatory bowel diseases such as ulcerative colitis and Crohn's disease, with some success.

Maintaining healthful colonies of bacterial flora can help to prevent the overgrowth of potentially harmful bacteria. Probiotic foods and supplements may help with this balance. (Rob3000/

Probiotics could have the potential to treat other autoimmune disorders, such as eczema (an inflammatory skin condition). Babies with a family history of eczema have shown a reduced risk of developing eczema if their mothers consumed foods or supplements with probiotics during pregnancy, and if babies received probiotic supplements early in life.

Probiotics are found in many foods, including dairy products such as yogurt and kefir. Fermented vegetable products such as pickles, sauerkraut, and kimchi (a Korean dish) also have active bacterial cultures. Probiotic drinks are popular in Japan. Foods containing helpful bacterial cultures have been consumed in many cultures for thousands of years and are considered generally safe. Probiotic supplements must deliver live bacteria to be helpful. Labels measure probiotic dosage in “colony-forming units” (CFUs). Supplements usually contain millions or billions of CFUs. Although probiotic supplements appear to be safe for most people, those with compromised immune systems should seek the guidance of a health care provider before taking probiotic supplements.

See Also: Fermentation and fermented foods; Large intestine; Microbiota and microbiome; Prebiotics

Further Reading
  • Bakalar, N. (2012, Nov 19). Probiotics linked to lowered diarrhea risk. New York Times. Retrieved from
  • Harvard Medical School. (2005). Health benefits of taking probiotics. Family Health Guide. Retrieved from
  • National Center for Complementary and Alternative Medicine. (2012). Oral probiotics: An introduction. Retrieved from
  • Sun, Y.; Zhang, M.; Chen, C.-C., et al. (2013). Stress-induced corticotropin-releasing hormone-mediated NLRP6 inflammasome inhibition and transmissible enteritis in mice. Gastroenterology, 144 (7), 1478-1487. doi:10.1053/j.gastro.2013.02.038.
  • Weil, A. (2014, December 15). Probiotics. Vitamin Library. Weil Lifestyle, LLC. Retrieved from
  • Barbara A. Brehm
    Rebecca Swartz
    Copyright © 2015 Barbara A. Brehm

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