lactobacillusEnglishOxford New American Dictionarylactobacillus /ˌlaktōbəˈsiləs, ˌlæktoʊbəˈsɪləs / ▸ noun (plural lactobacilli /-ˈsiˌlī, -ˈsiˌlē/) Biology a rod-shaped bacterium which produces lactic acid from the fermentation of carbohydrates. Genus Lactobacillus; nonmotile Gram-postive bacteria.