In a previous report (1978, Proc. Nat. Acad. Sci. USA 75, 3479-3483), we showed that recipient strains of Streptococcusfaecalis excrete a heat-stable substance (sex pheromone) which induces donor cells carrying certain conjugative plasmids to become adherent, generating the cell-to-cell contact necessary for plasmid transfer. Since donors themselves could be induced to aggregate or "clump" by recipient filtrates, the substance was referred to as "clumping-inducing agent" (CIA). In this report, we present a simplified assay for CIA and determine the level of activity in filtrates prepared at various stages of growth. We also present evidence that recipient cells produce multiple pheromones, each specific for donors harboring a particular class of plasmids. Whereas a recipient that acquires a conjugative plasmid no longer produces the corresponding CIA, it still produces CIAs specific for donors with different conjugative plasmids. In addition, an analysis of 100 clinical isolates of S. faecalis showed that drug-resistant strains are significantly more likely to respond to and produce CIA activities than drug-sensitive strains. A model is discussed describing the relationships of sex pheromones to the mating process.

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