Department of Clinical Pathophysiology University of Florence
A probiotic is defined classically as a live microbial food ingredient that is beneficial to health by improving the intestinal microbial balance.(1) Many studies have pointed out the beneficial effect of probiotics and their ability to promote gut barrier function, give maturational signals for the gut-associated lymphoid tissues, and balance the generation of pro- and anti-inflammatory cytokines, thereby creating healthy interactions between the host and microbes in the gut that are needed to keep inflammatory responses regulated but concomitantly readily primed.
An effective probiotic should: exert a beneficial effect on the host; be nonpathogenic and nontoxic; contain a large number of viable cells; be capable of surviving and metabolising in the gut; remain viable during storage and use; and have good sensory properties.
For human adult use, live microbial food includes fermented milk products and over-the-counter preparations containing lyophilised bacteria. The microorganisms involved are usually lactic acid producers such as lactobacilli (L. acidophilus, L. johnsonii, L. casei, L. gasseri and L. reuteri) and bifidobacteria (B. bifidum, B. longum, B. infantis), but may also include noninvasive coliforms and even nonbacterial organisms such as Saccharomyces boulardii.(2) A probiotic preparation may contain one or several different strains.
Viable and biologically active microorganisms are usually required at the target site in the host, so it is essential that probiotics can withstand the host’s natural barriers against ingested bacteria. Lactobacilli and bifidobacteria resist gastric acid, bile salts and pancreatic enzymes, adhere to intestinal mucosa and readily colonise the intestinal tract.
Lactic acid bacteria inhibit the in-vitro growth of many enteric pathogens, including Salmonella typhimurium, Staphylococcus aureus, Escherichia coli, Clostridium perfrigens and Clostridium difficile. S. boulardii, a patented yeast preparation, is used in many countries as a preventive and therapeutic agent for diarrhoea and gastrointestinal (GI) disorders caused by administration of antimicrobial agents for conditions unrelated to the GI tract.(3) S. boulardii possesses many properties that make it a potential probiotic agent – it inhibits the growth of several microbial pathogens in vivo and in vitro; its optimum temperature is 37°C; it survives transit through the GI tract; and it is unaffected by antibiotic therapy.
While there are hundreds of published studies describing the use of probiotics to prevent and treat GI disorders, only a few have been conducted with sufficient subjects, proper controls and statistical analysis. However, evidence that probiotics are useful therapeutically or prophylactically is available for certain disorders.
Gut barrier functions
The intestinal flora protects humans against infection, and its disturbance can increase susceptibility to infection. The GI tract provides a protective interface between the internal environment and the constant challenge from food-derived antigens and microorganisms in the external environment. As a result, the GI barriers control antigen transport and the generation of immunological phenomena in the gut. The gut microflora is an important constituent in the intestine’s defence barrier, and it has been shown that the gut microflora elicits specific immune responses at local and systemic level.(4) Moreover, the gut flora is shown to induce and maintain oral tolerance in experimental animal models, and intestinal colonisation acts as an important antigenic stimulus for the maturation of the gut-associated lymphoid tissue.
In addition to their effect on nonimmunologic gut defence, which is characterised by stabilisation of gut microflora, probiotics have also been shown to enhance the humoral immune response and promote the intestine’s immunological barrier. There have been several recent reports describing the effect of lactobacilli on IgA production. L. casei, L. acidophilus and yogurt enhance the number of IgA- producing plasma cells in a dose-dependent manner.(5) This increased secretion of IgA was sufficient to prevent enteric infection. In another study, mice fed lactic acid bacteria had increased splenocyte proliferation in response to mitogens for T- and B-cells.(6)
Perhaps the most intriguing aspect of probiotic modulation of immune response is its effect on cytokine production. Cytokines and their regulation of the immune system have been studied intensively, and several in-vitro and in-vivo studies have shown that cytokine production by the cells of the immune system can be altered by probiotic use. Oral therapy with Lactobacillus rhamnosus GG was shown to reduce elevated fecal concentration of TNF-a in patients with atopic dermatitis and cow milk allergy.(7) Paradoxically, ingestion of lactobacilli in fermented milk products or as live-attenuated bacteria was shown to potentate interferon-gamma production by peripheral blood mononuclear cells.(8) Interferon-gamma can promote uptake of antigens in Peyer’s patches, where specific IgA-committed cells are generated. Therefore ingestion of probiotics may stabilise the immunologic barrier of the gut mucosa by reducing the generation of local proinflammatory TNF-alpha and by reinforcing the systemic production of interferon-gamma with a physiologic protective effect on the gut.
Probiotics are also able to modify the structure of potentially harmful antigens and alter their immunogenicity. L. rhamnosus has the ability to hydrolyse casein, and its administration reduced the production of interleukin-4 in mononuclear cells of atopic infants with cow milk allergy.(9)
Genetically engineered bacteria may extend the scope of probiotic action. A recent study engineered the food-grade bacterium Lactococcus lactis to produce the anti- inflammatory cytokine IL-10.(10) When it was fed to different murine models of bowel inflammation, the IL-10-producing bacteria had a significant therapeutic effect. This approach has the advantage of organ-specific targeted drug therapy and avoids the inconvenience and hazards of parental administration.
Probiotics in GI disease
Antibiotic-associated diarrhoea (AAD) results from a microbial imbalance that leads to a decrease in the endogenous flora. Clostridium difficile and Klebsiella oxytoca contribute to the occurrence of AAD and play a role in the pathogenesis of colonic lesions. Three studies showed that oral administration of S. boulardii can decrease the risk and shorten the duration of AAD.(11) The therapeutic efficacy of other probiotics in AAD is not as well established.
Lactobacillus shows some promise as treatment for viral intestinal infections in infants. Daily administration of Lactobacillus GG shortened duration of diarrhoea in hospitalised children. The effect was even more significant when only the rotavirus-positive patients were analysed. (12) Beneficial effects of administration of L. acidophilus and S. boulardii were demonstrated in treatment of diarrhoea after pelvic irradiation and also in patients with HIV-related chronic diarrhoea.
Helicobacter pylori gastroenteritis
H. pylori colonisation of the gastric mucosa is strongly associated with chronic gastritis, duodenal and gastric ulcer and some malignancies. Lactobacillus is an antagonist to H. pylori in vitro, but attempts to eradicate H. pylori in vivo with probiotics have failed so far. However, a significant reduction in urease activity was reported in patients treated with a supernatant of L. johnsonii LA1 associated with omeprazole. (13)
Inflammatory bowel disease (IBD)
The rationale for considering probiotics as a therapeutic strategy for IBD is based on the persuasive evidence indicating that intestinal bacteria involved in the pathogenesis of IBD and the implication of normal flora in epithelial turnover, mucosal blood flow, smooth muscle function and mucosal immune response. The most compelling evidence is derived from animal models. Particularly noteworthy was the finding that the strain of Lactobacillus salivarius reduced the rate of progression from inflammation to colon cancer in IL-10 knockout mice compared with nonprobiotic-fed controls.(14) In humans, two studies using a nonpathogenic strain of E. coli (serotype O6:K5:H1) as a probiotic have shown a significant efficiency in maintenance of remission of ulcerative colitis.(15)
In Crohn’s disease (CD), an open trial of approximately 20 patients with mild-to-moderate active disease showed that probiotics could effectively offset the need for corticosteroid therapy in over 70% of patients.(16) A double-blind controlled study of the efficacy of S. boulardii on CD symptoms showed a significant reduction in the frequency of bowel movements and in disease activity.(17)
Promising results were obtained in pouchitis, a complication of ileal reservoir surgery occurring in 10–20% of patients who undergo surgical treatment for chronic ulcerative colitis. Bacteria overgrow in the pouch results in degradation of the mucus overlaying the epithelial cells. Investigators have postulated that Lactobacillus GG may be an effective therapeutic agent for pouchitis because it does not demonstrate mucus-degrading properties. Impressive results have been reported using a cocktail of probiotic organisms in a controlled trial. Forty patients in remission received either placebo or the probiotic cocktail (VLS3) consisting of viable lyophilised bacteria including different strains of lactobacilli, bifidobacteria and Streptococcus salivarius thermophilus. After nine months, the relapse rate in the probiotic group was significantly lower (15%) compared with the placebo group (100%).(18)
Lactose intolerance and sucrase-isomaltase deficiency
It is well established that people with lactose maldigestion experience better digestion and tolerance of the lactose in yogurt than of that in milk. The importance of the viability of lactic acid bacteria was speculated as pasteurisation reduced the observed digestibility. The mechanism involved the digestion of lactose in the gut lumen by the lactase contained in yogurt bacteria and the slower transit time of yogurt compared with milk. In clinical practice, the replacement of milk with fermented dairy products allows for better digestion and decreases diarrhoea and other symptoms of lactose intolerance. An enhanced digestion of a sucrose load was shown in infants with sucrose deficiency when they consumed S. cerevisiae, a yeast that contains the enzyme sucrase.(19)
Increased levels of ammonia in patients with liver failure are one of the mechanisms involved in the pathogenesis of hepatic encephalopathy. It has been postulated that probiotics can decrease the intestinal urease activity involved in the production of ammonia. Patients treated with L. acidophilus and neomycin show a greater decrease in faecal urease activity than patients treated with neomycin alone,(20) which corresponded to lower serum ammonia levels and to a significant improvement in the clinical status of patients.
Probiotics and colon cancer
Several evidences have demonstrated that the normal intestinal flora can influence carcinogenesis by producing enzymes (glycosidase, beta-glucoronidase, azoreductase, nitroreductase) that transform precarcinogens into active carcinogens. Different investigators have shown that probiotics (L. acidophilus, L. casei) may decrease the faecal concentration of these enzymes.(21) Whether this leads to a reduced incidence of cancer is unknown, but epidemiologic studies suggest that consumption of fermented dairy products may have some protective effects against large-colon adenomas or cancer.(22) Clinical studies are ongoing to study the effects of probiotics and prebiotics in subjects with colonic adenomas.
The development of successful probiotic products will depend on proof of the probiotic effects and the development of foods that harbour high numbers of viable organisms at the time of consumption. The probiotic approach (ie, therapeutically consuming beneficial microorganism cultures of healthy human microflora) holds great promise for the prevention and treatment of clinical conditions associated with impaired gut mucosal barrier functions. Even though many health benefits are associated with the use of probiotics, research into the mechanisms by which these cultures exert their effects is still at an early stage, and many unresolved issues can be answered only by well-designed and controlled clinical trials. Future studies need to be conducted involving the comparison between agents and doses, cost–benefit analyses, and efforts to determine the exact mechanisms by which these agents yield their effects.
- Fuller R. Probiotics in man and animals. J Appl Bacteriol 1989;66:365-78.
- Holzapfel H, et al. Taxonomy and important features of probiotic micro-organisms in food and nutrition. Am J Clin Nutr 2001;73 Suppl:365S-73S.
- Boddy AV, et al. Influence of antibiotics on the recovery and kinetics of Saccharomices boulardii in rats. Pharm Res 1991;8:796-800.
- Shanahan F. Nutrient tasting and signalling mechanisms in the gut. Mechanisms of immunologic sensation and intestinal contents. Am J Physiol Gastrointest Liver Physiol 2000;278:G191-6.
- Perdigon G, et al. Immune system stimulation by probiotics. J Dairy Sci 1995;78:1597-606.
- De Simone C, et al. The role of probiotics in modulation of immune system in man and in animals. Int J Immunother 1993;9:23-8.
- Majarmaa H, Isolauri E. Probiotics: a novel approach in the management of food allergy. J Allergy Clin Immunol 1997;99:179-86.
- Halpern GM, et al. Influence of long term yogurt consumption in young adults. Int J Immunother 1991;7:205-10.
- Sutas Y, Hurme M, Isilauri E. Down regulation of anti CD3 antibody-induced IL-4 production by bovine caseins hydrolysed with lactobacillus GG-derived enzymes. Scand J Immunol 1996;43:687-9.
- Steidler L, et al. Treatment of murine colitis by Lactococcus lactis secreting IL-10. Science 2000;289:1352-5.
- Elmer GW, Surawicz CM, McFarland LV. Biotherapeutic agents. A neglected modality for the treatment and prevention of selected intestinal and vaginal infection. JAMA 1996;275:870-6.
- Isolauri E, et al. Oral bacteriotherapy for viral gastroenteritis. Dig Dis Sci 1994;39:2595-600.
- Michetti P, et al. Effect of whey-based culture supernatant of Lactobacillus acidophilus (johnsonii) La1 on Helicobacter pylori infection in humans. Digestion 1999;60:203-9.
- Madsen KL, et al. Lactobacillus species prevents colitis in interleukin 10 gene-deficient mice. Gastroenterology 1999;116:1107-14.
- Rembacken BJ, et al. Non-pathogenic E. coli versus mesalazine for the treatment of ulcerative colitis: a randomized trial. Lancet 1999;354:635-9.
- Malin M, et al. Promotion of IgA immune response in patients with Crohn’s disease by oral bacteriotherapy with Lactobacillus GG. Ann Nutr Metab 1996;40:137-45.
- Plein K, Hotz J. Therapeutic effect of Saccharomices boulardii of mild residual symptoms in a stable phase of Crohn’s disease with special respect to chronic diarrhea – a pilot study. Z Gastroenterol 1993;31:129-34.
- Shanahan F. Inflammatory bowel disease. Immunodiagnostics, immuno-therapeutics, and ecotherapeutics. Gastroenterology 2001;120:622-35.
- De Vrese M, et al. Probiotics – compensation for lactase insufficiency. Am J Clin Nutr 2001:73 Suppl:421S-9S.
- Loguercio C, Del Vecchio Blanco C, Coltorti M. Enterococcus lactic acid bacteria strain SF68 and lactulose in hepatic encephalopathy: a controlled study. J Int Med Res 1987;15:335-43.
- Brady JL, Gallaher DD, Busta FF. The role of probiotic cultures in the prevention of colon cancer. J Nutr 2000;130:410S-4S.
- Rafter JJ, The role of lactic acid bacteria in colon cancer prevention. Scand J Gastroenterol 1995;30:497-502.
University of Florence
Department of Clinical
Italian Society of Gastroenterology