Choline-binding protein D (CbpD) in Streptococcus pneumoniae is essential for competence-induced cell lysis

Streptococcus pneumoniae is an important human pathogen that is able to take up naked DNA from the environment by a quorum-sensing-regulated process called natural genetic transformation. This property enables members of this bacterial species to efficiently acquire new properties that may increase their ability to survive and multiply in the human host. We have previously reported that induction of the competent state in a liquid culture of Streptococcus pneumoniae triggers lysis of a subfraction of the bacterial population resulting in release of DNA. We have also proposed that such competence-induced DNA release is an integral part of natural genetic transformation that has evolved to increase the efficiency of gene transfer between pneumococci. In the present work, we have further elucidated the mechanism behind competence-induced cell lysis by identifying a putative murein hydrolase, choline-binding protein D (CbpD), as a key component of this process. By using real-time PCR to estimate the amount of extracellular DNA in competent relative to noncompetent cultures, we were able to show that competence-induced cell lysis and DNA release are strongly attenuated in a cbpD mutant. Ectopic expression of CbpD in the presence or absence of other competence proteins revealed that CbpD is essentially unable to cause cell lysis on its own but depends on at least one additional protein expressed during competence.