Genetic Transformation of Streptococcus sanguis (Challis) with Cryptic Plasmids from Streptococcus ferus

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Abstract
By using the basic methodology initially published by Kretschmer et al., one was able to introduce phenotypically cryptic plasmids from S. ferus (formerly S. mutans ssp. ferus) into S. sanguis by genetic transformation. In this system, the entry of the cryptic plasmids is selected indirectly. This is effected with transforming DNA mixtures in which the cryptic plasmid DNA is present in an approximate 10-fold molar excess with respect to a plasmid (pVA1) known to confer erythromycin resistance. Under such conditions, 5-10% of the pVA1-containing erythromycin-resistant transformants were cotransformed with cryptic plasmid DNA. pVA1 may be selectively eliminated by growth of its S. sanguis host strain at 42.degree. C, enabling the construction of isogenic strains with and without S. ferus cryptic plasmids. Comparative physiological studies of such strains failed to reveal any plasmid-conferred phenotypes in S. sanguis. With this procedure one was able to physically separate 2 small cryptic plasmids (2.4 .times. 106 and 2.8 .times. 106 daltons) of S. ferus. Although these plasmids were found naturally to exist in a single S. ferus host, they were able to replicate independently of one another in S. sanguis. Restriction enzyme fingerprinting indicated that these plasmids did not share a common ancestry.