Lactobacillus acidophilus is a commensal bacterium – it lives in the gastrointestinal tract of humans and other animals without causing disease. The bacteria metabolize lactose, a sugar found in milk and dairy products. Byproducts of the metabolism are lactic acid and antimicrobial products that prevent the growth of pathogenic bacteria. The bacteria are often found in yogurts to aid in dairy consumption and are present in alternative milk products to alleviate lactose intolerance. Too little acidophilus can result in impaired digestion.
Humans are initially colonized by the bacterium hours after birth. Babies born by Cesarean section are not exposed to the same bacteria as those born vaginally, but those that breast feed also have the opportunity to be exposed to and colonized by the bacteria. There are other beneficial bacteria that are also gained through this same mechanism. Colic and reflux is less common in babies who are properly colonized by the commensal bacteria.
After antibiotic therapy, some patients are administered acidophilus supplements to replace the bacteria and prevent opportunistic bacterial infections like Clostridium difficile. Acidophilus is also often used to treat thrush, a fungal infection of the mouth, in infants. Probiotics, beneficial bacteria used medicinally to influence the intestinal flora, have become a popular supplement market, but are often already found in foods or are provided naturally from the environment.
Lactobacillus acidophilus is also the chief means by which the human body obtains vitamin K, via synthesis in the intestines. This vitamin is a necessary cofactor for blood clotting and healthy bone tissue turnover (see the Colorado State University page on the vitamin K cycle for more information). The bacterium also aids in B vitamin production, such as niacin, folic acid, B6, B12, and biotin. Acidophilus also helps recycle the amino acids in bile and normalize cholesterol.
Some evidence indicates the benefit of acidophilus in preventing and treating yeast and other fungal infections in women, diarrhea, and urinary tract infections during antibiotic therapy. However, the American Cancer Society points out that there have been no definitive human studies on the use of acidophilus to preventing or treating cancer and animal studies have been contradictory. For a complete list of what is known and not yet confirmed for treatments using Lactobacillus acidophilus, see MedlinePlus.
Please note that though acidophilus is considered a probiotic, there is some risk of too much bacteria in the digestive tract and it is recommended that any supplementation outside of food consumption be done under the direction of a doctor.
Natural sources of acidophilus and probiotics are yogurt containing live cultures. Prebiotics, or fructo-oligosaccharides, are non-digestible sugar chains that enhance the growth of probiotic cultures. Foods containing these sugars include bananas, onion, asparagus, artichoke, and garlic.