Lactobacillus acidophilus is a member of Lactobacillus sp., a group of Gram-positive non-sporing facultative or anaerobic rods that are indigenous inhabitants of the human intestine and vagina. L. acidophilus are found mainly in the distal end of the small intestine. The primary purpose of L. acidophilus, and other indigenous microflora, is to reinforce the protective barrier of the mucosal surfaces and prevent the attachment of pathogenic microorganisms and entrance of allergens. They accomplish this through several mechanisms of action including competing with pathogens for epithelial space and nutrients, and maintaining the epithelial surface at a low, acidic pH that is inhibitory to pathogenic bacteria. Lactic acid bacteria, including L. acidophilus, metabolize carbohydrate and produce lactic acid, acetic acid, hydrogen peroxide, and short-chain fatty acids such as propionate and acetate. These byproducts of carbohydrate metabolism are associated with many of L. acidophilus's protective effects. Lactic acid and the short chain fatty acids help reduce the pH of the mucosal surface, while hydrogen peroxide is antagonistic to pathogenic bacteria and yeasts. L. acidophilus bacteria also produce inhibitory substances called bacteriocins that kill microbes and bacteria. Together, these byproducts of glucose fermentation make an unfriendly environment for growth of less favorable and potentially pathogenic microorganisms, such as staphylococci Pseudomonas and Salmonella.

L. acidophilus also produce lactase, the enzyme that breaks down milk sugar (lactose) to glucose and galactose. Many humans do not produce lactase endogenously and may benefit from L. acidophilus supplementation. L. acidophilus may also stimulate the immunoglobulin secretory IgA, which helps prevent infection at mucosal surfaces. L. acidophilus foods and supplements are referred to as a "probiotics" because they beneficially affect the host animal by improving its microbial balance. Certain strains of L. acidophilus, such as L. acidophilus ADH, have been shown to survive in gastric juice and colonize the epithelial surface, better than others.

• Acidophilus milk
• Yogurt (with live L. acidophilus)
• Other fermented dairy products

Commercial products typically contain L. acidophilus (DDS-1 or other strains) with one to five billion viable organisms per serving (capsule or tablet). Powders may contain ten billion cells per gram.

• Freeze-dried granules
• Freeze-dried powders
• Freeze-dried capsules
• Liquid L. acidophilus preparations (refrigerated)

Antibiotic-induced dysbiosis: Antibiotic treatment may destroy beneficial bacteria in the colonic epithelium, creating an environment for pathogenic organisms to flourish. L. acidophilus has been used successfully to prevent and treat bacterial imbalances caused by antibiotic therapy. Lactobacillus (L. acidophilus and L. bulgaricus acidophilus) preparations have been shown to help prevent ampicillin-induced diarrhea in adults. A probiotic fermented food mixture enriched with L. acidophilus was shown to inhibit growth of Shigella dysenteriae, Salmonella typhosa, and E. coli in vitro. The mixture helped to arrest diarrhea in mice suffering from E. coli-induced diarrhea immunomodulation.

Urogenital and gastrointestinal infections: Weekly intravaginal Lactobacillus therapy has been shown to reduce the recurrence rate of lower urinary tract infections. L. acidophilus strains that are resistant to Nonoxynol-9, a spermicide contraceptive that kills beneficial vaginal flora, may help prevent recurrent cystitis in women. L. acidophilus La1 has been shown to cause acid-independent, partial suppression of Helicobacter pylori growth in vitro and in vivo in H. pylori-infected volunteers.

Lactose intolerance: Numerous studies have shown that yogurt (with lactic acid bacteria) improves lactose absorption in lactose intolerant individuals. Yogurt appears to be more beneficial than unfermented acidophilus milk, although some positive results have been observed in individuals using unfermented L. acidophilus milk. Efficacy may be dependent on the type of acidophilus strain used in a product. In one study, three of four L. acidophilus strains tested (B, N1, and E) caused significant improvement, while L. acidophilus ATCC 4356 had no effect on lactose absorption.

Vaginal infections: Yogurt, enriched with live L. acidophilus, has been shown to prevent recurring bacterial vaginosis and candidal vaginitis in humans. Daily ingestion of enriched yogurt results in higher levels of L. acidophilus colonization in the rectum and vagina and decreased candidal colonization.

AIDS: AIDS patients may benefit from L. acidophilus's immunomodulatory action and ability to enhance antimicrobial resistance. Ingestion of L. acidophilus strain La1 has been shown to enhance phagocytic activity of peripheral blood leukocytes in humans. Immunocompromised patients with candida overgrowth may also benefit from L. acidophilus therapy.

• Prevention or treatment of diarrhea: one to two billion viable cells per day; some experts recommend up to ten billion cells per day.
• Vaginal infections: eight ounces of yogurt daily; or an oral supplement containing one to two billion live organisms, daily.
• Cystitis: one to two capsules or tablets (intravaginally) nightly for two weeks.
• Maintaining normal intestinal flora: one to ten billion viable cells per day, continuously.

Mild gastrointestinal disturbance may occur in some individuals (not on antibiotic therapy) who exceed more than one to two billion L. acidophilus cells per day.

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In vitro, an isolated strain of Lactobacillus acidophilus rapidly degraded sulphasalazine and phthalylsulphathiazole; the metabolic byproducts, sulphapyridine and sulphathiazole, can contribute to renal toxicity (Pradham and Majumdar 1986). More research is needed to confirm these effects in vivo.

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