Lactic Acid Bacteria: Lactococcus lactis

Lactococcus lactis is a member of the mesophilic group of bacteria and an important constituent of many industrial and artisanal starter cultures. The principal role of lactococci in dairy fermentations involves the metabolism of lactose to lactic acid and the conversion of milk proteins into flavor compounds. Lactococci are Gram-positive, catalase-negative, facultatively anaerobic, nonmotile, and non-spore-forming. Lc. lactis cultures found in dairy fermentations are classified as subspecies cremoris, lactis, and lactis biovar diacetylactis. Many traits in lactococci that render these microorganisms suitable for dairy fermentations are encoded on plasmids. Indeed, traits such as lactose utilization, casein breakdown, bacteriophage resistance, bacteriocin production, and exopolysaccharide production have all been associated with extrachromosomal plasmid DNA. Plasmids isolated from lactococci range in size from 3 to 130 kb, have a G+C content of 30-40. mol%, and vary in function and distribution with most strains carrying between 4 and 7 plasmids per cell. To date, three lactococcal genomes have been completely sequenced, and they range in size from 2.37 to 2.53 Mbp with a G+C content of approximately 35. mol%. The difference in G+C content between lactococcal plasmids and genomes suggests that many lactococcal plasmids may be recent additions to the genome enabling their host to grow efficiently in milk.