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
