Lactobacillus acidophilus (New Latin 'acid-loving milk-bacterium') is a species of gram positive bacteria in the genus Lactobacillus.. acidophilus is a homofermentative, microaerophilic species, fermenting sugars into lactic acid, and grows readily at rather low pH values (below pH 5.0) and has an optimum growth temperature of around 37 °C (99 °F).