Conjugal transfer of genetic information in group N streptococci

Streptococcus lactis strains ML3 and C(2)O and S. lactis subsp. diacetylactis strains DRC3, 11007, and WM(4) were found to transfer lactose-fermenting ability to LM0230, an S. lactis C2 lactose-negative (Lac(-)) derivative which is devoid of plasmid deoxyribonucleic acid (DNA). Lactose-positive streptomycin-resistant (Lac(+) Str(r)) recombinants were found when the Lac(+) Str(s) donor was mixed with Lac(-) Str(r) LM0230 in solid-surface matings. Transduction and transformation were ruled out as the mechanism of genetic exchange in strains ML3, DRC3, 11007, and WM(4), nor was reversion responsible for the high number of Lac(+) Str(r) recombinants. Furthermore, chloroform treatment of the donor prevented the appearance of recombinants, indicating that transfer of lactose-fermenting ability required viable cell-to-cell contact. Strain C(2)O demonstrated transduction as well as conjugation. Transfer of plasmid DNA during conjugation for all strains was confirmed by demonstrating the presence of plasmid DNA in the transconjugants by using agarose gel electrophoresis. In some instances, a cryptic plasmid was transferred in conjunction with the lactose plasmid by using strains DRC3, 11007, and WM(4). In S. lactis C2 x LM0230 matings, the Str(r) marker was transferred from LM0230 to C2, suggesting conjugal transfer of chromosomal DNA. The results confirm conjugation as another mechanism of genetic exchange occurring in dairy starter cultures.