Microorganisms establish themselves in or on nonsterile parts of the body shortly after birth and continue to colonize the human body throughout one’s life.1

The flora that permanently live in or on the body are called resident flora.1 Most of these bacteria reside in the gut and outnumber normal human cells 10 to 1.2

The most common area in the body for beneficial microbes to live is in the gut, though they also can be found in the lungs, mouth, skin, urinary tract, and vagina.3

The relationship between host (the human body) and resident flora is usually a symbiotic one. Rather than cause disease, the flora protects the body against disease-causing organisms.1 Under certain circumstances, however, resident flora may cause disease. This typically occurs with injury, surgery, use of antibiotics, or a weakened immune system.1

The metabolic activities of the resident flora in the large intestine are important for health and include converting fibers to short-chain fats and manufacturing vitamins.2

The gut flora is very sensitive to diet, and an imbalance can be linked to numerous diseases, such as Alzheimer, colon cancer, depression, heart disease, metabolic syndrome, and type 2 diabetes.2 Probiotics can help restore the correct balance.2

Probiotics are a combination of live bacteria, protozoa, viruses, and/or yeast that are similar to resident flora. These good microbes help keep bodies healthy and working well.4

Many types of bacteria and yeast can be considered probiotics, each with different benefits. The most common bacteria and yeast are:
  • Bifidobacterium. This probiotic bacterium can be found in some dairy products.4 There about 30 species of Bifidobacterium. This is the most common resident microbe found in the colon.5
  • Lactobacillus. This is the most common probiotic bacteria. Lactobacillus can be found in yogurt and other fermented foods.4 More than 50 species of lactobacilli exist.5 This bacterium has been used to prevent and treat many conditions and diseases.5
  • Saccharomyces boulardii. This is the most common yeast found in probiotics.4

Although supplements can be acquired OTC, the ideal way to get probiotics is through the diet. Food and drinks that contain probiotics include as follows:
  • Cheese. Cheddar, gouda, and mozzarella all contain probiotics. It is important, however, to look for active and live cultures on the food labels.6
  • Kefir. This is a fermented milk drink made by adding kefir grains, cultures of lactic acid bacteria, and yeast, to cow’s or goat’s milk. Kefir is a better source of probiotics than yogurt.6
  • Kimchi. This spicy Korean side dish contains fermented vegetables, usually cabbage, and is flavored with a mix of seasonings. Kimchi contains lactic acid bacteria, including Lactobacillus kimchii.6
  • Kombucha. This black or green tea drink is fermented with various bacteria and yeast. It is claimed to be beneficial, but more research is needed.6
  • Miso. This Japanese seasoning paste is made with fermented soybeans, which are soybeans fermented with salt and a fungus called Aspergillus oryzae or koji.6
  • Natto. This fermented soybean product contains Bacillus subtilis. Natto is a staple in Japanese kitchens, usually mixed with rice and eaten for breakfast, and it has a high amount of vitamin K2.6
  • Pickles. Pickled cucumbers are fermented in salty water and sometimes spices. Pickles are high in vitamin K. Pickles fermented in vinegar do not have probiotic properties.6
  • Sauerkraut. This cabbage is fermented by using lactic acid bacteria. The probiotic form of sauerkraut is unpasteurized, as the pasteurizing process kills the good bacteria. Other health benefits of sauerkraut include antioxidants lutein and zeaxanthin, a high-fiber content, iron, magnesium, sodium, and vitamins B, C, and K.6
  • Tempeh. This fermented high-protein, soybean meat substitute tastes earthy, nutty, or similar to a mushroom, and comes in patty form. Tempeh contains vitamin B12, which is found primarily in animal products.6
  • Traditional buttermilk. The noncultured leftover liquid from making butter is the only kind of buttermilk that contains probiotics. Traditional buttermilk also contains calcium, phosphorus, riboflavin, and vitamin B12. It is traditionally consumed in India, Nepal, and Pakistan. The cultured buttermilk in the United States does not have probiotic properties.6
  • Yogurt. Made from fermented milk, not all yogurt contains live probiotics. In some cases, processing can kill these probiotics. Choose yogurt with active or live cultures.6

Study results have shown probiotics may help in the treatment of many gastrointestinal conditions, including Clostridium difficile infection, constipation, diarrhea caused by cancer treatment, diverticular disease, inflammatory bowel disease, irritable bowel syndrome, and traveler’s diarrhea.7

Certain conditions in infants may be improved with probiotics, including infant colic and sepsis and necrotizing enterocolitis. Always consult a pediatrician before giving a child of any age probiotics.7

Dental disorders, such as dental caries and periodontal diseases, may decrease with the use of probiotics.7

Allergies, such as atopic dermatitis and allergic rhinitis, may be prevented with the use of probiotics.7

Other conditions that may improve with probiotic use include acne, upper respiratory infections, and urinary tract infections.7

Taking antibiotics, especially for long periods of time, may cause antibiotic-associated diarrhea. This occurs when antibiotics kill off some of the resident flora in the gut, resulting in an imbalance that allows harmful bacteria to thrive.2 Study results show that probiotics can help restore that balance and keep harmful bacteria in check.2

The microbes used as probiotics occur naturally in the human body and are generally considered safe. In some people, probiotics can trigger an allergic reaction. They may also cause bloating, diarrhea, flatulence, and mild stomach upset. These adverse effects, aside from allergic reactions, usually resolve within a few days.3

More serious adverse effects can occur in people with compromised immune systems or severe illnesses.7 These include antibiotic resistance and infections.7

The FDA regulates probiotics in a unique way. Depending on the product’s intended use, the agency may regulate a product as a dietary supplement, drug, or food ingredient.7 Dietary supplements do not require FDA approval and are not required to be effective or safe. If a probiotic is to be marketed as a drug for treating certain diseases or disorders, it must prove effective and safe for its intended use through clinical trials and be approved by the FDA.7
Kathleen Kenny, PharmD, RPh, has more than 25 years of experience as a community pharmacist and is a freelance clinical medical writer based in Colorado Springs, Colorado.

REFERENCES 1. Bush LM, Schmidt SE. Resident Flora. Merck Manual. Updated July 2020. Accessed August 28, 2020. https://www.merckmanuals.com/home/infections/biology-of-infectious-disease/resident-flora 2. Gunnars C. Probiotics 101: a simple beginner’s guide. Healthline. November 13, 2018. Accessed August 15, 2020.https://www.healthline.com/nutrition/probiotics-101 3. Probiotics. Cleveland Clinic. March 9, 2020. Accessed August 15, 2020. https://my.clevelandclinic.org/health/articles/14598-probiotics 4. Health benefits of taking probiotics. Harvard Health Publishing. Updated April 2020. Accessed September 25, 2020. https://www.webmd.com/digestive-disorders/ what-are-probiotics#1 5. Harbolic BK, What are probiotics? MedicineNet. Accessed August 15, 2020. https://www.medicinenet.com/probiotics/article.htm 6. Palsdottir H. 11 probiotic foods that are super healthy. Healthline. August 28, 2020. Accessed September 1, 2020. https://www.healthline.com/nutrition/11-super-healthy-probiotic-foods 7. Probiotics: what you need to know. National Institutes of Health. Updated August 2019. Accessed August 15, 2020. https://www.nccih.nih.gov/health/probiotics-whatyou-need-to-know