Molecular diversity and relationship within Lactococcus lactis, as revealed by randomly amplified polymorphic DNA (RAPD)

Lactococcus lactis strains are widely used in industrial dairy fermentation s. Conventional phenotypic tests have been used for years to classify membe rs of this species into two subspecies, lactis and cremoris, and play a key role in the choice of strains to be used in particular cheese fermentation s. DNA hybridisation techniques have also been used for strain classificati on, giving rise to two genome homology groups. However, results showed disc repancies between the two methods of classification. We applied the randoml y amplified polymorphic DNA fingerprinting (RAPD) technique to resolve prev ious contradictions in lactococcal classifications. Unlike usual RAPD metho ds, we use three primers to classify 113 strains and integrate the resultin g information by a digitised programme used for this purpose. Our analysis revealed three major RAPD groups, designated G1, G2 and G3. G1 and G3 conta in strains of the lactis subspecies, and G2 contains strains of the cremori s subspecies, as previously defined by phenotypic characteristics. Moreover , group G1 corresponds to one genome homology group, and groups GZ and G3 c orrespond to the second one. The taxonomic structure within L. lactis is th erefore unusual: two distinct genetic groups of strains show indistinguisha ble phenotypes, while conversely, two phenotypically distinct groups are ge netically homologous. We hypothesize that a subfamily of the subsp. lactis group gave rise to the cremoris subspecies.