Investigation into thermal adaptation mechanisms of Streptococcus pneumoniae and its impact on pneumococcal pathogenicity

Temperature is a critical and ubiquitous environmental signal that influences the growth, virulence, and survival of diverse microbial species. The human pathogen Streptococcus pneumoniae is a commensal microorganism, whose ecological niche is the human nasopharynx, and becomes pathogenic after spreading to other niches such as lung, blood stream, or brain. During progression from colonisation to invasion, the pneumococcus must contend with temperature fluctuations that can influence its phenotypic character. This study demonstrated that temperature has a significant impact on pneumococcal phenotype, such as growth, biofilm formation, metabolism, and virulence factor generation. To understand the thermal adaptation mechanisms employed by this microbe, the pneumococcal transcriptome was obtained by microarray analysis at 34°C, 37°C, or 40°C. The analysis showed that 6% (132) and 5% (120) of genes were differentially expressed at 34ºC and 40ºC, respectively, at mid-exponential growth phase by 2-fold relative to the expression at 37ºC. Interestingly, the genes upregulated in one temperature were downregulated at another temperature. For further analysis, I selected seven genes (gdhA, ciaR, comD, merR2, SPD_0132/0133, SPD_1711, and SPD_1651) that were expressed differentially at different temperatures and characterized the mutants using different experimental techniques such as growth profile analysis, cell size measurement, biofilm formation, capsule synthesis, and for the production of virulence proteins. I found that loss of the selected genes led to attenuation at 40°C in all assays comparing to the wild type strain. It was particularly interesting to find that glutamate dehydrogenase (gdhA, SPD_1158) is highly active at 40°C rather than 34°C, suggesting that GdhA is a thermostable protein. In vivo analysis showed that gdhA-null strain grown at 40°C was less virulent in G. mellonella acute infection model, indicating that GdhA is required for pneumococcal virulence at high temperatures. In conclusion, temperature fluctuation has impact on transcriptional and phenotypic profiles of S. pneumoniae.