Lactococcus lactis is a Gram-positive bacterium used extensively in the production of buttermilk and cheese, but has recently also become famous as the first genetically modified organism to be used alive for the treatment of human disease.
L. lactis cells are cocci that group in pairs and short chains, and, depending on growth conditions, appear ovoid with typically 0.5 - 1.5 µm in length.
L. lactis does not produce spores (nonsporulating) and are not motile (nonmotile). They have a homofermentative metabolism and have been reported to produce exclusively L(+) lactic acid. However, reported D(-) lactic acid can be produced when cultured at low pH. The capability to produce lactic acid is one of the reasons why
L. lactis is one of the most important microorganisms in the dairy industry. Based on its history in food fermentation
L. lactis has the GRAS status (Generally Regarded As Safe) with few case reports of being an opportunistic pathogen.
L. lactis is of crucial importance for manufacturing dairy products, such as buttermilk and cheeses. When
L. lactis ssp. lactis is added to milk, the bacterium uses enzymes to produce energy molecules (ATP), from lactose. The byproduct of ATP energy production is lactic acid. The lactic acid produced by the bacterium curdles the milk that then separates to form curds, which are used to produce cheese. Other uses that have been reported for this bacterium include the production of pickled vegetables, beer or wine, some breads, and other fermented foodstuffs, such as soymilk kefir, buttermilk, and others.
L. lactis is one of the best characterized low GC Gram positive bacteria with detailed knowledge on genetics, metabolism and biodiversity.
L.lactis is mainly isolated from either the dairy environment or plant material. Dairy isolates are suggested to have evolved from plant isolates through a process in which genes without benefit in the rich medium milk were either lost or down-regulated. This process, also called genome erosion or reductive evolution is also described in several other lactic acid bacteria. The proposed transition from the plant to the dairy environment was reproduced in the laboratory through experimental evolution of a plant isolate that was cultivated in milk for a prolonged period. Consistent with the results from comparative genomics (see references above) this resulted in L. lactis losing or down-regulating genes which are dispensable in milk and the up-regulation of peptide transport.