Competence-induced cells of Streptococcus pneumoniae lyse competence-deficient cells of the same strain during cocultivation.

Several streptococcal species are able to take up naked DNA from the environment and integrate it into their genomes by homologous recombination. This process is called natural transformation. In Streptococcus pneumoniae and related streptococcal species, competence for natural transformation is induced by a peptide pheromone through a quorum-sensing mechanism. Recently we showed that induction of the competent state initiates lysis and release of DNA from a subfraction of the bacterial population and that the efficiency of this process is influenced by cell density. Here we have further investigated the nature of this cell density-dependent release mechanism. Interestingly, we found that competence-induced pneumococci lysed competence-deficient cells of the same strain during cocultivation and that the efficiency of this heterolysis increased as the ratio of competent to noncompetent cells increased. Furthermore, our results indicate that the lysins made by competent pneumococci are not released into the growth medium. More likely, they are anchored to the surface of the competent cells by choline-binding domains and cause lysis of noncompetent pneumococci through cell-to-cell contact.