Conjugal Transfer of Lactose-Fermenting Ability Among Streptococcus cremoris and Streptococcus lactis Strains

Streptococcus cremoris C3 was found to transfer lactose-fermenting ability to LM2301, a Streptococcus lactis C2 lactose-negative streptomycin-resistant (Lac Str) derivative which is devoid of plasmid deoxyribonucleic acid (DNA); to LM3302, a Lac erythromycin-resistant (Ery) derivative of S. lactis ML3; and to BC102, an S. cremoris B(1) Lac Ery derivative which is devoid of plasmid DNA. S. cremoris strains R1, EB(7), and Z8 were able to transfer lactose-fermenting ability to LM3302 in solid-surface matings. Transduction and transformation were ruled out as mechanisms of genetic transfer. Chloroform treatment of donor cells prevented the appearance of recombinant clones, indicating that viable cell-to-cell contact was responsible for genetic transfer. Transfer of plasmid DNA was confirmed by agarose gel electrophoresis. Transconjugants recovered from EB(7) and Z8 matings with LM3302 exhibited plasmid sizes not observed in the donor strains. Transconjugants recovered from R1, EB(7), and Z8 matings with LM3302 were able to donate lactose-fermenting ability at a high frequency to LM2301. In S. cremoris R1, EB(7), and Z8 matings with LM2301, streptomycin resistance was transferred from LM2301 to the S. cremoris strains. The results confirm genetic transfer resembling conjugation between S. cremoris and S. lactis strains and present presumptive evidence for plasmid linkage of lactose metabolism in S. cremoris.