Lactobacillus acidophilus (Neo-Latin 'acid-loving milk-bacillus') is a rod-shaped, Gram-positive, homofermentative, anaerobic microbe first isolated from infant feces in the year 1900.

L. acidophilus has antagonistic effects on the growth of Staphylococcus aureus, Escherichia coli, Salmonella typhimurium, and Clostridium perfringens.

[6] In the L. acidophilus species, the N-terminal region shows high amino acid variability along with low sequence homology (31-72%).

Because of its use as a probiotic in milk, a study done by the American Journal of Dairy Science examined the nutrient requirements of L. acidophilus in an effort to increase its low growth rate.

[1] Genetic analysis also revealed that all L. acidophilus strains contained at least 15 families of glycosyl hydrolases, which are the key enzymes in carbohydrate metabolism.

Each of the 15 GH families were involved in metabolizing common carbohydrates, such as glucose, galactose, fructose, sucrose, starch, and maltose.

[medical citation needed] Nearly all Lactobacillus species have special mechanisms for heat resistance which involves enhancing the activity of chaperones.

In another experiment highlighted by the American Dairy Science Association, viable cell counts only showed a significant reduction after exposure to NaCl concentrations of 7.5% or higher.

[13] In Lactobacillus acidophilus, which can be found in the gastrointestinal tract, quorum sensing is important for bacterial interaction when considering biofilm formation and toxin secretion.

[22] Research has shown that the presence of L. acidophilus can produce a variety of probiotic effects in humans, such as acting as a barrier against pathogens, assisting in lactose digestion, enhancing immune response, and reducing cholesterol level.

[23] Another study conducted at Maranatha Christian University studied the impact of L. acidophilus cell free supernatants (a liquid medium containing the metabolites produced by microbial growth)[24] on the growth pattern of Salmonella typhi, the microbe associated with Typhoid fever.

The study showed that the presence of L. acidophilus metabolites significantly inhibited the growth curves displayed by S. typhi,[25] supporting the idea that L. acidophilus presence has a positive impact on the species makeup of a gut microbial community, providing the organism with intestinal health benefits.

[27] There is also some evidence supporting the use of a symbiotic gel (containing L. acidophilus) in treating gastrointestinal symptoms in patients who had received a hemodialysis treatment.

Reduced serum cholesterol levels, stimulated immune response, and improved lactic acid digestion are all probiotic effects associated with intestinal L. acidophilus presence.

L. acidophilus was also effective in reducing Streptococcus mutans levels in saliva, as well as decreasing risk factors associated with the development of nonalcoholic fatty liver disease.

Although probiotics are generally safe, when they are used by oral administration there is a small risk of passage of viable bacteria from the gastrointestinal tract to the blood stream (bacteremia), which can cause adverse health consequences.

[29] Some people, such as those with a compromised immune system, short bowel syndrome, central venous catheters, cardiac valve disease and premature infants, may be at higher risk for adverse events.