Television commercials marketing certain brands of yogurt as being “guaranteed” to help with symptoms of irritable bowel syndrome, chronic constipation, and other gastrointestinal maladies have become common. One of the main bacteria that are contained in these products is Lactobacillus acidophilus. In New Latin, the name means acid-loving milk bacterium.1 Often included in probiotic products, L acidophilus has been studied for its possible impact on a multitude of human ailments. L acidophilus is believed to be beneficial because it produces vitamin K and lactase.


L acidophilus was originally isolated from the human gut in 1900.1 In the decades since, it has been used to treat ailments ranging from irritable bowel syndrome to high cholesterol and Lyme disease.2 In humans, L acidophilus is found in what is presumed its natural habitat, the gut and vagina, which are usually at or near the bacteria’s optimum growth temperature (99°F) and pH (<5.0).3 Its function is to act as gatekeeper by preventing other pathogens from gaining entrance, presumably because of the hostile pH environment. 


L acidophilus has been studied extensively for its potential in treating gastric Helicobacter pylori infections. The proposed mechanism of action is alteration of the environment in the gastric lining, including reduction of pH, direct interference with H pylori replication, and adhesion restriction.4

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Despite multiple clinical trials, there is no consensus about the mechanism of action, as results are mixed and difficult to interpret.5 One finding that has been noted across trials is that adding L acidophilus to standard therapy for H pylori infection, consisting of high-dose antibiotics and a proton pump inhibitor, reduces side effects of treatment such as diarrhea and stomach pain.6