A Simple and Rapid Method for Genetic Transformation of Lactic Streptococci by Electroporation

An electroporation procedure for the plasmid-mediated genetic transformation of intact cells of Streptococcus cremoris and Streptococcus lactis was performed. Ten different strains were transformed. The method was simple and rapid and yielded transformant colonies in 14 to 24 h. The method was optimized for S. lactis LM0230, and transformation frequencies of between 1 x 10 and 5 x 10 transformants per mug of purified plasmid (pMU1328) were achieved routinely. The optimized procedure involved lysozyme treatment of cells. Transformation of LM0230 occurred at comparable frequencies with pLS1 (4.4 kilobase pair [kbp]), pMU1328 (7.4 kbp), and pAMbeta1 (26.5 kbp). Plasmid DNA isolated from transformants had not undergone detectable deletions or rearrangements. Transformation was possible with plasmid DNA which was religated after restriction endonuclease digestion. Phage DNA-dependent transfection of S. lactis LM0230 and S. lactis C6 was also achieved.