The involvement of CiaR and the CiaR-regulated serine protease HtrA in thermal adaptation of .

The temperature can vary according to the host tissue and the response to infection. has evolved mechanisms to survive these temperature differences, but neither the consequences of different temperatures for pneumococcal phenotype nor the genetic basis of thermal adaptation are known in detail. In our previous study [16], we found that CiaR, which is a part of two-component regulatory system CiaRH, as well as 17 genes known to be controlled by CiaRH, were identified to be differentially expressed with temperature. One of the CiaRH-regulated genes shown to be differentially regulated by temperature is for the high-temperature requirement protein (HtrA), coded by SPD_2068 (). In this study, we hypothesized that the CiaRH system plays an important role in pneumococcal thermal adaptation through its control over . This hypothesis was evaluated by testing strains mutated or overexpressing and/or , in and assays. The results showed that in the absence of , the growth, haemolytic activity, amount of capsule and biofilm formation were considerably diminished at 40 °C only, while the cell size and virulence were affected at both 34 and 40 °C. The overexpression of in the ∆ background reconstituted the growth at all temperatures, and the haemolytic activity, biofilm formation and virulence of ∆ partially at 40 °C. We also showed that overexpression of in the wild-type promoted pneumococcal virulence at 40 °C, while the increase of capsule was observed at 34 °C, suggesting that the role of changes at different temperatures. Our data suggest that CiaR and HtrA play an important role in pneumococcal thermal adaptation.