Lactobacillus acidophilus
Lactobacillus acidophilus is one of several bacteria in the genus Lactobacillus. It is in some countries sometimes used commercially together with Streptococcus salivarius and Lactobacillus delbrueckii ssp. bulgaricus in the production of acidophilus-type yogurt. Also known as the Boas-Oppler bacillus after Ismar Isidor Boas, a German gastroenterologist (1858-1938) and Bruno Oppler (-1932) a German physician from Breslau.
Lactobacillus acidophilus gets its name from lacto- meaning milk, -bacillus meaning rod-like in shape, and acidophilus meaning acid-loving. This bacterium thrives in more acidic environments than most related microorganisms (pH 4-5 or lower) and grows best at 45 degrees Celsius. L. acidophilus occurs naturally in the human and animal gastrointestinal tract, mouth, and vagina.
L. acidophilus ferments lactose into lactic acid, like many (but not all) lactic acid bacteria. Certain related species (known as heterofermentive) also produce ethanol, carbon dioxide, and acetic acid this way. L. acidophilus itself (a homofermentative microorganism) produces only lactic acid. Like many bacteria, L. acidophilus can be killed by excess heat, moisture, or direct sunlight.

# Health benefits
Some strains of L. acidophilus may be considered a probiotic or "friendly" bacteria. These types of healthy bacteria inhabit the intestines and vagina and protect against some unhealthy organisms. The breakdown of nutrients by L. acidophilus produces lactic acid, hydrogen peroxide, and other byproducts that make the environment hostile for undesired organisms. L. acidophilus also tends to consume the nutrients many other microorganisms depend on, thus outcompeting possibly harmful bacteria in the digestive tract. During digestion, L. acidophilus also assists in the production of niacin, folic acid, and pyridoxine.  L. acidophilus can assist in bile deconjugation, separating amino acids from bile acids, which can then be recycled by the body.
Some research has indicated L. acidophilus may provide additional health benefits, including improved gastrointestinal function, a boosted immune system, and a decrease in the frequency of vaginal yeast infections. Some people report L. acidophilus provides relief from indigestion and diarrhea. A University of Nebraska study found that feed supplemented with L. acidophilus and fed to cattle resulted in a 61% reduction of Escherichia coli 0157:H7. Research has indicated L. acidophilus may be helpful reducing serum cholesterol levels.
L. acidophilus is part of the normal vaginal flora. The acid produced by L. acidophilus in the vagina helps to control the growth of the fungus Candida albicans, helping to prevent vaginal yeast infections. The same beneficial effect has been observed in cases of oral or gastrointestinal Candidiasis infections. Certain spermicides and contraceptive creams can kill L. acidophilus in the vagina, clearing the path to possible yeast infections.
Antibiotics taken orally will also kill beneficial bacteria like L. acidophilus. After a course of antibiotic therapy, patients are occasionally instructed to take an L. acidophilus treatment in order to recolonize the gastrointestinal tract.
L. acidophilus is often sold in health stores in pill or powder form as a nutritional supplement. Research on the nutritional benefits of taking L. acidophilus supplements is inconsistent and inconclusive. Most such claims boil down to a link between L. acidophilus and a possible decrease in the incidence of certain diseases, including yeast infections, gastrointestinal disorders, and a weakened immune system. Most researchers agree further study is needed before substantiating many of these claims.