Beschreibung:
<jats:p>Competence for genetic transformation in the human pathogen <jats:italic>Streptococcus pneumoniae</jats:italic> is a transient physiological property. A competence‐stimulating peptide, CSP, was recently identified as the processed product of the <jats:italic>comC</jats:italic> gene. As conflicting results have been reported regarding CSP autoinduction, we monitored the CSP‐induced expression of <jats:italic>comCDE</jats:italic> in derivatives of strain R6 using <jats:italic>comC</jats:italic>::<jats:italic>lacZ</jats:italic> fusions. Autoinduction was demonstrated in this genetic background. The kinetics of CSP‐induced transcription of <jats:italic>comCDE</jats:italic> and of a late competence‐induced (<jats:italic>cin</jats:italic>) operon were compared. While the <jats:italic>comCDE</jats:italic> mRNA level was highest 5 min after CSP addition then decreased, maximal <jats:italic>cin</jats:italic> expression required 10 min exposure to CSP. Transformation frequencies paralleled <jats:italic>cin</jats:italic> expression. After 20 min exposure to CSP, both mRNAs disappeared almost completely, providing evidence for an intrinsic mechanism for shutting off CSP signal transduction. Investigation of spontaneous competence development in mixed cultures indicated that transformation of wild‐type cells was delayed in the presence of CSP non‐producers, consistent with a direct role of CSP in quorum sensing. The effect of varying inoculum size on the timing of competence development was investigated. While competence developed in wild‐type cultures at a similar critical density, about OD<jats:sub>550</jats:sub> = 0.15, a mutant lacking the three oligopeptide‐binding lipoproteins transformed at a 50‐fold reduced cell density. The latter effect was mimicked in a strain harbouring a duplication of <jats:italic>comC</jats:italic>. Altogether, these results suggest that CSP does not accumulate passively in pneumoccal cultures, but that <jats:italic>comCDE</jats:italic> basal expression can be modulated.</jats:p>