Transformation of Lactococcus by Electroporation

Lactococcus lactis strains are the principal organisms used in cheese-making, and their performance is of vital importance to the quality of the cheese. A goal for the dairy industry has always been to improve strains of L. lactis and to stabilize beneficial traits. In recent years, genetic engineering has become a useful tool for manipulating L. lactis. A critical step in this work is the introduction of DNA into the cells. The first successful transformations of L. lactis employed protoplast transformation in the presence of polyethylene glycol (1). This method was not very reproducible and worked only for a few strains. More recently, electroporation proved to be an alternative to protoplast transformation in L. lactis. Harlander (2) was the first to employ electroporation in this species. Later work indicated that the cell wall is a physical barrier to the entering DNA not only during protoplast transformation, but also during electroporation. Powell et al. (3) used lysozyme treatment to obtain an increase in transformation efficiency, and van der Lelie et al. (4) obtained cells competent for electrotransformation by growing them in the presence of high concentrations of threonine to weaken their cell walls. However, the methods were not very efficient, and a number of strains could not be transformed.