Health professionals have known for generations that bacteria play both good and bad roles in the human body. But until the past few years, medical literature has said little about the need to ensure a homeostatic balance of bacteria in the gut. While the thought of intentionally ingesting bacteria is generally repugnant, it can help avert a potentially harmful decrease in essential intestinal flora.


Most metabolic processes involving bacteria occur in the digestive tract. Each person’s mix of bacterial species varies, but the most common groups are Lactobacillus and Bifidobacterium.1 Because these bacteria aid digestion by helping to break  down foods and form waste, they are known as probiotics.

Many foods, such as yogurt, miso, and some juices and soy beverages, contain probiotics naturally. These microorganisms survive processing, accounting for the “live, active culture” phrase you may see, particularly on yogurt labels.1

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Ideally, a person’s normal food intake will keep gut bacteria in balance, but the equilibrium can be disrupted. Antibiotics are the most common cause. Although they are designed to kill pathogenic bacteria, many antibiotics are broad-spectrum, i.e., they kill other species as well, including Lactobacillus and Bifidobacterium.

The result is an overgrowth of remaining bacterial species and poor nutrient absorption. Clostridium difficile is the best known of these hardy species. It is responsible for an estimated 15%-25% of all diarrheas that are a common side effect of antibiotic use.2

Evidence of effectiveness

Researchers at Imperial College in London conducted a randomized, double-blind study to test whether probiotics could mitigate the diarrheal effect of antibiotics. The 135 hospitalized patients drank a mixture of Lactobacillus casei, Lactobacillus bulgaricus, and Streptococcus thermophilus or a placebo beverage twice daily during their antibiotic regimens and for one week afterward. None of the 82 participants in the treatment arm developed C. difficile infection, compared with nine of the 53 participants (16.9%) taking placebo.2

An earlier meta-analysis on the effect of administering a probiotic therapy to alleviate antibiotic-associated diarrhea reviewed seven studies involving a total of nearly 900 patients. In the pooled data, the incidence of antibiotic-associated diarrhea fell from 23% in the placebo group to 13% in the treatment group. Statistically, those results equated to one fewer case of antibiotic-associated diarrhea for every 10 patients using the probiotic treatment.3

Another diarrhea scenario concerned 69 children hospitalized with rotavirus infection. In this trial, the diarrheal phase was reduced by 20% as compared with placebo when an oral Lactobacillus mixture was ingested twice daily for five days. The length of hospitalization was reduced by 48%.4

Bacterial balance also seems to affect irritable bowel syndrome (IBS). Two studies randomized a total of 100 patients who had been diagnosed with IBS by the Rome criteria. Participants received either probiotics or placebo. In the four-week study, 95% of treated patients reported improvement in symptoms compared with only 15% of placebo patients.5 In the 12-month trial, treated patients also reported more symptom abatement (52% vs. 24%).6 

Finally, physiologists have defined several specific roles for probiotic bacteria in the immune system to protect it and to enhance its efficiency. The two most important are a barrier function that facilitates and regulates nutrient absorption and an immunomodulatory effect that actually stimulates the production of interleukin-10 and regulatory T cells.7 Small studies have even shown support of CD4 cell counts in people with HIV/AIDS.8

Safety and dose

Probiotics are ingested in foods, such as active-culture yogurts, or as dietary supplements. Because they have the potential to alter immune systems, probiotics should be used cautiously in young children, elderly people, and anyone with a compromised immune system.9 Dose determinations vary widely, and the range most closely matching those in clinical trials is recommended. For children, that would be 5 billion to 10 billion colony-forming units (CFU) per day; for adults, it would be 10 billion to 20 billion CFU per day.10

Probiotic therapy for digestive conditions such as IBS may require time to find the food, supplement, and dose that work for the individual.

Allergies to actual bacteria have not been documented, but they can be triggered by the carrier agent (i.e., the specific food in which the probiotics are found). Side effects, if they occur, tend to be mild and digestive, such as gas or bloating, if they occur. Often they are dose-dependent.9


Many primary-care patients with IBS, diarrhea, or immune-support needs could benefit from this very simple intervention. It’s hard to think of a patient who wouldn’t benefit from a stronger immune system and healthier bowel function.


1. National Center for Complementary and Alternative Medicine. An introduction to probiotics.
2. Hickson M, D’Souza AL, Muthu N, et al. Use of probiotic Lactobacillus preparation to prevent diarrhoea associated with antibiotics: randomised double blind placebo controlled trial. BMJ. 2007;335:80.
3. Cremonini F, Di Caro S, Nista EC, et al. Meta-analysis: the effect of probiotic administration on antibiotic-associated diarrhoea. Aliment Pharmacol Ther. 2002;16:1461-1467.
4. Rosenfeldt V, Michaelsen KF, Jakobsen M, et al. Effect of probiotic Lactobacillus strains in young children hospitalized with acute diarrhea. Pediatr Infect Dis J. 2002;21:411-416.
5. Niedzielin K, Kordecki H, Birkenfeld B. A controlled, double-blind, randomized study on the efficacy of Lactobacillus plantarum 299V in patients with irritable bowel syndrome. Eur J Gastroenterol Hepatol. 2001;13:1143-1147.
6. Nobaek S, Johansson ML, Molin G, et al. Alteration of intestinal microflora is associated with reduction in abdominal bloating and pain in patients with irritable bowel syndrome. Am J Gastroenterol. 2000;95:1231-1238.
7. Madsen K. Probiotics and the immune response. J Clin Gastroenterol. 2006;40:232-234.
8. Anukam KC, Osazuwa EO, Osadolor HB, et al. Yogurt containing probiotic Lactobacillus rhamnosus GR-1 and L. reuteri RC-14 helps resolve moderate diarrhea and increases CD4 count in HIV/AIDS patients. J Clin Gastroenterol. 2008;42:239-243.
9. Lacy BE. Prebiotics, probiotics, & antibiotics: controversies in the treatment of IBS. PowerPoint presentation: Annual GI review course at the University of Maryland School of Medicine; 2008; Baltimore.
10. Kligler B, Cohrssen A. Probiotics. Am Fam Physician. 2008;78:1073-1078.

All electronic documents accessed April 9, 2009.