Lactococcus lactis
Ashley Muehler

Lactococcus lactis is a spherical-shaped, gram-positive cocci bacterium that groups in pairs and short chains.  It was originally classified under Streptococcus lactis due to its chain forming ability, but was reclassified in 1985.  The bacterium is non-sporulating and non motile.  When cultured in the lab on nutrient agar, it appears bright orange.  The bacterium is mesophilic and is commonly found on plant and animal surfaces.  L. lactis stays dormant on plant surfaces waiting to be ingested by animals, particularly cows, and then transferred to the gastrointestinal tract where it becomes active and multiplies intensively.

L. lactis is a facultative anaerobe that converts carbohydrates into lactic acid by lactic acid fermentation.  The lactic acid fermentation can produce L-lactic acid and D-lactic acid at low pH.  When added to milk, the bacterium uses enzymes to produce ATP from lactose the byproduct of which is lactic acid.  Lactic acid curdles the milk that then separates to form curds which are used to produce cheese and whey.  It is therefore used for industrial production of fermented dairy products such as milk, cheese, yogurt, soymilk, cottage cheese, cream cheese, sour cream, and buttermilk.  L. lactis is used in the early stages of production of such chesses as Brie, Camembert, Cheddar, Colby, Gruyere, Parmesan, Roquefort, Monterey Jack, Gouda, Edam, Muenster, Feta, Blue, Gorgonzola, and Havarti.  Not only is L. lactis involved in the production of dairy products but the manufacture of such things as pickled vegetables, beer, wine, sausages, and breads as well.  L. lactis is comprised of two subspecies with very few phenotypic and genotypic differences.  Lactococcus lactis lactis is preferred for making soft cheeses and produces glutamate decarboxylase.  Lactococcus lactis cremoris is preferred for making hard cheeses.  It does not contain glutamate decarboxylase due to a frameshift mutation resulting in nonfunctioning proteins.

Lactococcus lactis is so important in the cheese making industry, there is petitioning to make it the Wisconsin State Microbe.  Although Wisconsin has lost its status as the number one dairy state, thanks to L. lactis they have maintained their position as the number one cheese-producing state.   Wisconsin is the leading manufacturer of L. lactis starter cultures in the U.S.  Bacteriophages can cause significant economic losses each year by preventing the bacteria from fully metabolizing the milk.  These phages include species 936, c2, and P2335.

The genome of L. lactis is comprised of a circular chromosome with 2,365,589 base pairs.  86% of the genome codes for proteins, 1.4% for RNA, and 12.6% are noncoding regions.  The bacterium requires glucose, arginine, methionine, glutamate, and valine for growth.  It can use fructose, galactose, glucosamine, glucose, lactose, maltose, mannitol, mannose, ribose, sucrose, and trehalose as carbon sources.  It does grow slower on fructose and mannitol.

Extensive research has been done on the metabolic pathway of the bacterium to increase its efficiency for dairy production as these are important food supplies for many people.  Improvements in activities and effectiveness have been reached by manipulating the environment of the bacterium.  Researchers are attempting DNA recombination to improve its survival and resistance to antibiotics.  Not only is L. lactis important in food production but the bacterium is also being looked at as a potential oral vaccine for developing countries against Streptococcus pneumoniaeL. lactis is treated to recombine vaccine strains so the cell can express the vaccine protein PspATIGR4.  Results from current research shows that it has more potential and safety in developing a vaccine for humans and should be considered for use against more pathogens.


*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.


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