lactobacillusEnglishOxford Dictionary of Englishlactobacillus /ˌlaktəʊbəˈsɪləs / ▸ noun (plural lactobacilli /ˌlaktəʊbəˈsɪlʌɪ/) Biology a rod-shaped bacterium which produces lactic acid from the fermentation of carbohydrates. Genus Lactobacillus; non-motile Gram-positive bacteria.