In general, Lactobacilli is the largest genus of the lactic acid bacteria group and includes over 50 species. Lactobacilli commonly inhabit the gastrointestinal (GI) tract, oral, and vaginal regions of humans and animals.
Lactobacilli have many important roles in industry. They contribute to the production of some cheeses, yogurt, and other products. The lactic acid produced by Lactobacilli inhibits the growth of other organisms and lowers the pH of the product in these products. The starter cultures for such products are carefully cultivated and maintained because their metabolic end products contribute to the flavor of the final food product. Additionally, some of Lactobacilli's metabolic reactions are intentionally manipulated to breakdown milk proteins during cheese production.
Early studies of L. acidophilus were performed on strains isolated from fecal material of humans, pigs and chickens. Since then L. acidophilus has been further characterized as a short Gram-positive rod (2-10μm), is homofermentative and has optimal growth at temperatures of 37˚C-42˚C. Of the Lactobacillus species, L. acidophilus is the most well known and is commercially distributed as a probiotic. The World Health Organization defines a probiotic as "live microorganisms which, when administered in adequate amounts, confer a health benefit on the host".
Further isolation and investigation into the physiological, biochemical, genetic, and fermentative properties have been widely explored in both humans and animals. The L. acidophilus strain, NCFM, was isolated from a human in 1970 and characterized at North Carolina State University. NCFM has been commercially available in the United States as a probiotic strain since the mid-1970s. NCFM is also used for formula, yogurt and fluid milk production.