Genetic and physiological analysis of conjugation in Streptococcus faecalis

In an effort to elucidate the mechanisms of conjugal plasmid transfer in S. faecalis, a genetic analysis of the sex pheromone-dependent tetracycline resistance plasmid pCF-10 was initiated. Rare transconjugants obtained from short matings with wild-type donors not exposed to sex pheromones were screened for increased donor potential in a subsequent mating. From this screening, a mutant plasmid, designated pCF-11, whose transfer functions are expressed in the absence of pheromone induction was isolated. Cells carrying pCF-11 spontaneously clump when grown in broth culture but do not excrete sex pheromones active against wild-type donors. In the course of initial experiments, it was observed that physiological conditions could affect plasmid transfer frequency. Therefore, a set of standardized optimal mating conditions was defined. The experiments carried out to determine these conditions revealed that a transient increase in transfer frequency of about 2 orders of magnitude occurred in early-exponential-phase donor cells. This peak of activity is independent of sex pheromone response, since it was observed with induced or uninduced donor cells carrying either pCF-11 or pCF-10.