Nasopharyngeal carriage is the reservoir from which most disease with Streptococcus pneumoniae arises. Survival as a commensal in this environment is likely to require a set of adaptations distinct from those needed to cause disease, some of which may be mediated by two-component signal transduction systems (TCSTS). We examined the contributions of nine pneumococcal TCSTS to the process of nasopharyngeal colonization by using an infant rat model. Whereas deletions in all but one of these systems have been associated previously with a high degree of attenuation in a murine model of pneumonia, only the CiaRH system was necessary for efficient carriage. Transcriptional analysis by using microarray hybridization identified a locus consisting of two adjacent genes, htrA and spoJ, that was specifically and strongly downregulated in a ⌬ciaRH-null mutant. A S. pneumoniae strain lacking the htrA gene encoding a putative serine protease, but not one lacking spoJ, showed decreased fitness in a competitive model of colonization, a finding consistent with this gene mediating a portion of the carriage deficit observed with the ⌬ciaRH strain.Streptococcus pneumoniae exists in nature both as a commensal on the mucosal surface of the human upper respiratory tract and as a pathogen causing disease in a wide range of sites, including the lung, middle ear, sinuses, blood, and meninges. Survival in these different host environments is likely to require adaptive responses, some of which may be mediated by twocomponent signal transduction systems (TCSTS). These signaling systems (reviewed in reference 27) are found in a wide range of bacteria where they have been shown to regulate diverse processes, including chemotaxis, nutrient utilization, surface adhesion, and the switch between aerobic and anaerobic metabolism. TCSTS consist of a sensor kinase that autophosphorylates in response to an environmental stimulus and a cognate response regulator to which an activated phosphate is transferred and that then mediates a downstream response, often acting as a DNA-binding protein to cause changes in gene expression.Two genomic surveys of S. pneumoniae have identified 13 pairs of genes encoding putative TCSTS as well as a single unpaired putative response regulator (12, 29). The ability of null mutants in these systems to cause disease has been previously examined in two studies of virulence. In a murine model of pneumonia developing after intranasal inoculation in which all of the TCSTS except ComDE were tested, deletions in eight of the systems resulted in an attenuation of at least 3 orders of magnitude (29). In contrast, a second study testing a collection of single-crossover mutants in a different genetic background showed no evidence of decreased virulence for any of the strains in a murine model of intraperitoneal infection in which the requirements of adaptation to the mucosal surface of the airway were circumvented (12). Despite the importance of nasal carriage both as the first step in the pathogenesis of pneumococcal disease and in...

Competence for genetic transformation in Streptococcus pneumoniae develops in response to accumulation of a secreted peptide pheromone and was one of the initial examples of bacterial quorum sensing. Activation of this signaling system induces not only expression of the proteins required for transformation but also the production of cellular chaperones and proteases. We have shown here that activity of this pathway is sensitively responsive to changes in the accuracy of protein synthesis that are triggered by either mutations in ribosomal proteins or exposure to antibiotics. Increasing the error rate during ribosomal decoding promoted competence, while reducing the error rate below the baseline level repressed the development of both spontaneous and antibiotic-induced competence. This pattern of regulation was promoted by the bacterial HtrA serine protease. Analysis of strains with the htrA (S234A) catalytic site mutation showed that the proteolytic activity of HtrA selectively repressed competence when translational fidelity was high but not when accuracy was low. These findings redefine the pneumococcal competence pathway as a response to errors during protein synthesis. This response has the capacity to address the immediate challenge of misfolded proteins through production of chaperones and proteases and may also be able to address, through genetic exchange, upstream coding errors that cause intrinsic protein folding defects. The competence pathway may thereby represent a strategy for dealing with lesions that impair proper protein coding and for maintaining the coding integrity of the genome.

Background:The pneumococcal HtrA protease represses competence selectively when the frequency of errors in protein synthesis is low. Results: HtrA digests both the bacterial competence-stimulating peptide (CSP) and unfolded proteins. Conclusion:The presence of proteins with exposed hydrophobic regions competitively reduces the degradation of CSP. Significance: This suggests a mechanism by which HtrA functions as a sensor for biosynthetic errors.