Lactococcus, from Latin lac, meaning "milk", and Ancient Greek κόκκος (kókkos), meaning "berry", is a genus of lactic acid bacteria that were formerly classified as Lancefield group N streptococci. They are known as homofermenters meaning that they produce a single product, lactic acid in this case, as the major or only product of glucose fermentation. Their homofermentative character can be altered by adjusting environmental conditions such as pH, glucose concentration, and nutrient limitation. They are gram-positive, catalase-negative, non-motile cocci that are found singly, in pairs, or in chains. The genus contains strains known to grow at or below 7˚C.
Taxonomy The type species of the genus is Lactococcus lactis. The List of Prokaryotic names with Standing in Nomenclature (LPSN) recognizes 16 species with validly published and correct names in Lactococcus:
Lactococcus intestinalis is treated as a later heterotypic synonym of L. muris. A genome-based revision divided the genus into two phylogenetic groups and introduced the genus Pseudolactococcus for one of them. Although Pseudolactococcus was validly published, LPSN currently treats it as a heterotypic synonym of Lactococcus.
Uses These organisms are commonly used in the dairy industry in the manufacture of fermented dairy products such as cheeses. They can be used in single-strain starter cultures, or in mixed-strain cultures with other lactic acid bacteria such as Lactobacillus and Streptococcus. Special interest is placed on the study of L. lactis subsp. lactis and L. lactis subsp. cremoris, as they are the strains used as starter cultures in industrial dairy fermentations. Their main purpose in dairy production is the rapid acidification of milk; this causes a drop in the pH of the fermented product, which prevents the growth of spoilage bacteria. The bacteria also play a role in the flavor of the final product. Lactococci are currently being used in the biotechnology industry. They are easily grown at industrial scale on whey-based media. As food-grade bacteria, they are used in the production of foreign proteins that are applied to the food industry.
Diseases Piscine lactococcosis is an acute hemorrhagic septicemic disease of freshwater and marine fish. It was historically attributed primarily to L. garvieae, but L. petauri and L. formosensis are also recognized as causative agents. These closely related species can be difficult to distinguish using routine phenotypic and biochemical methods, and genomic analyses have reassigned some isolates formerly identified as L. garvieae. Lactococcal infections have also been reported in humans and other terrestrial animals. Human infections attributed to L. garvieae have included infective endocarditis, bacteremia, meningitis, and peritonitis, although some historical identifications may require reassessment following the recognition of L. petauri and L. formosensis.
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