RESUMO
A modified medium which distinguished between citrate-fermenting and non-citrate-fermenting species of lactic streptococci within 48 h was developed. In addition, the occurrence of citrate-negative variants in citrate-positive populations of Streptococcus lactis subsp. diacetylactis could be detected.
RESUMO
It has been previously observed that loss of plasmid pGK4101 occurred concomitantly with loss of lactose-fermenting ability in Streptococcus lactis subsp. diacetylactis 18-16. Transfer of this 41-megadalton plasmid to LM0230, a lactosenegative (Lac) strain of S. lactis, required cell-to-cell contact and resulted in a conversion of LM0230 to the Lac phenotype. This confirms the linkage of lactose-fermenting ability to the 41-megadalton plasmid in S. lactis subsp. diacetylactis and, in addition, demonstrates transfer by a process resembling conjugation in the group N streptococci.
RESUMO
The use of Streptococcus diacetylactis as a flavor producer in dairy fermentations is dependent upon its ability to produce diacetyl from citrate. Treatment of S. diacetylactis strains 18-16 and DRC1 with acridine orange resulted in the conversion of approximately 2% of the DRC1 population and 20% of the 18-16 population to citrate negative, which is indicative of the involvement of plasmid deoxyribonucleic acid (DNA). Growth in the presence of acridine orange also resulted in the appearance of 2% lactose-negative derivatives in S. diacetylactis 18-16 and 99% lactose-defective, proteinase-negative derivatives in S. diacetylactis DRC1. Cesium chloride-ethidium bromide equilibrium density gradients of cleared lysate material from each strain revealed the presence of covalently closed circular DNA. Samples of this covalently closed circular DNA were subjected to agarose gel electrophoresis to determine the plasmid composition of each strain. S. diacetylactis 18-16 was found to possess six plasmids, of approximately 41, 28, 6.4, 5.5, 3.4, and 3.0 megadaltons (Mdal). S. diacetylactis DRC1 contained six plasmids, of approximately 41, 31, 18, 5.5, 4.5, and 3.7 Mdal. Variants of S. diacetylactis 18-16 which failed to produce acetoin plus diacetyl from citrate (citrate negative) were missing a 5.5-Mdal plasmid. Lactose-negative mutants of the same strain were devoid of a 41-Mdal plasmid. Lactose-defective, proteinase-negative mutants of S. diacetylactis DRC1 were missing a 31-Mdal plasmid. The citrate-negative mutants of S. diacetylactis DRC1 isolated in this study did not possess a 5.5-Mdal plasmid. Thus, we have evidence that there is a correlation between the ability to utilize citrate and the presence of a 5.5-Mdal plasmid. A relationship was also noted between lactose fermentation and proteinase activity and plasmid DNA in S. diacetylactis.
