Lactococcus is a genus of Gram-positive, facultatively anaerobic, coccoid lactic-acid bacteria of the family Streptococcaceae, found in milk, dairy products, plants and the mammalian gut. Strains of Lactococcus lactis subsp. lactis and subsp. cremoris are the principal starters for cheese (Cheddar, Gouda, cottage) and fermented milks, driving rapid acidification, curd formation, flavour (diacetyl, amino-acid catabolites) and texture in dairy fermentation. Phage attack on lactococcal starters is the classic fermentation failure in dairies, managed by strain rotation and phage-resistant cultures; some strains also produce the preservative nisin.