Lactococcus lactis is a Gram-positive bacteria used extensively in the production of buttermilk and cheese. L. lactis are cocci that group in pairs and short chains,and depending on growth conditions appears ovoid with typically 0.5 - 1.5 µm in length. L. lactis do not produce spores (non-sporulating) and are not motile (non-motile). Cultured in the laboratory, L. lactis colonies appear bright orange on nutrient agar. They have a homo-fermentative metabolism and have been reported to produce exclusively L(+) lactic acid . However, reported that D(-) lactic acid can be produced when cultured at low pH. The capability to produce lactic acid is one of the reasons why Lactococcus lactis is one of the most important micro-organisms involved in the dairy industry. Generally, it has been considered as an opportunistic pathogen . Even though, the number of clinical cases associated with infections by these microorganisms has increased in the last decade in both humans and animals. L. lactis is a bacterium which has a crucial importance for manufacturing dairy products such as buttermilk, yogurt or cheese. When L. lactis ssp. lactis is added to milk, the bacterium uses enzymes to produce energy molecules, called 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 and whey .

Other uses that have been reported for this bacteria include the production of pickled vegetables, beer or wine, some breads and other fermented food-stuffs such as soymilk kefir, buttermilk, ... . Nowadays, researchers believe that understanding the physiology and genetic make-up of this bacterium will provide food manufacturers as well as the pharmaceutical industry with invaluables benefits

Cheese production

L. lactis subsp. lactis (formerly Streptococcus lactis) is used in the early stages for the production of many cheeses including Brie, Camembert cheese, cheddar, Colby, Gruyère, Parmesan, and Roquefort.

The use of L. lactis in dairy factories is not without issues. Bacteriophages specific to L. lactis cause significant economic losses each year by preventing the bacteria from fully metabolizing the milk substrate. Several epidemiologic studies showed that the phages mainly responsible for these losses are from the species 936, c2 and P335.

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