Stress response and pathogenicity in Streptococcus pneumoniae

The pathogen Streptococcus pneumoniae encounters different levels of oxygen during the infection cycle including colonisation, pneumonia, bateraemia and meningitis. These different anatomical niches require high levels of genome changes to sense and respond to those external environmental stimuli. The bacterial gene expression is known to be affected by oxygen, and it must react properly for survival and for developing invasive pneumococcal desiseases (IPDs). Microarray techniques have allowed scanning the whole pneumococcal genome during growth in different tensions of oxygen mimicking in vivo conditions. It was found that oxygenated growth conditions have significantly elevated several key virulence genes. This was further confirmed with qRT-PCR for a selection of genes implicated in pathogenicity. Moreover, post-transcriptional stages have been also investigated such as protein production, biofilm formation, biological activities and adherence assays for several virulence factors performed under the effect of presence or absence of oxygen. The data illustrate that 420 out of 2,236 genes (17 % of the entire TIGR4 genome) were differentially expressed in the presence of oxygen compared to its absence. 262 genes (11 %) were over-expressed when pneumococci were grown in oxygenated conditions relative to transcriptional profile in anaerobic growth conditions, indicating the magnitude of roles played by oxygen on pneumococcal gene expression. Anaerobic growth of TIGR4 showed down-regulation of 158 genes (7 %). Oxygen modulates induction of ply, pspC and other seven genes involved in pili structuring subunits (rlrA, rrgA, rrgB and rrgC) and assembling enzymes (srtB, srtC and srtD). This may suggest that the pneumococcal population grown under atmospheric environment is equipped with greater capability to progress IPDs compared to anaerobically grown bacteria. In addition to this, pneumococcal adhesion in vitro for TIGR4 grown in oxygenated or anaerobic growth conditions revealed a significant increase in those grown in oxygenated growth conditions, indicating that oxygen may play a key role in bacterial-host attachment. Interestingly, ablation of pspC has resulted in similar adhesion percentages of TIGR4 grown under both conditions, oxygenated and anaerobic. Furthermore, several genes involved in metabolism were up-regulated in oxygenated environment, particularly transporters, which are considered highly important for a bacterium that lacks an electron transport chain, catalase and tricarboxylic acid. Additionally, the results showed phenotypic characterisation and changes in cells morphology from pneumococcal growth curves for several strainswith different genome backgrounds grown under different levels of oxygen concentrations. Further investigation of the pathogen biology revealed differences in pneumolysin production and activity. These findings highlight that virulence genes expression is induced once the micro-organism is exposed to oxygenated environment, and data analysis has demonstrated potential links between pneumococcal metabolism and their ability to cause diseases