genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
genes repressed (Garzoni et al. 2007) or induced (this study), saeRS repressed (Garzoni et al.
2007) or induced (this study).
Clearly, the comparability is limited due to experimental differences, which underlines the
importance of performing global molecular studies (transcriptome and proteome profiling) in
different models to finally achieve a complete picture of the characteristics of host-pathogen
interactions.
Such different models include e. g. other epithelial cell lines, like the S9 cells used in this study,
but can also be extended to immune cells. Such a study was performed by Voyich and colleagues
in 2005. The authors analyzed the gene expression profile of different S. aureus strains upon
phagocytosis by human neutrophils (Voyich et al. 2005). Again, several experimental parameters
differed between the studies of Voyich and colleagues (2005), Garzoni and colleagues (2007), and
this study, partly resulting from the very different host cell type employed by Voyich and
coworkers. The authors described immune evasion mechanisms of S. aureus, which are initiated
by transcriptional changes. First, the authors observed induction of stress response genes, e. g.
superoxide dismutases sodA and sodM and several other genes involved in counteracting the
influence of reactive oxygen intermediates. An as strong response was not observed in this study,
which can be explained by the innate function of neutrophils to kill pathogens, which is not given
for epithelial cells. Thus, staphylococci internalized in S9 cells probably encounter less harmful
molecules than staphylococci after uptake by neutrophils. The induction of TCA cycle genes after
phagocytosis by neutrophils (Voyich et al. 2005) corresponds with results from this study using S9
cells. Voyich and coworkers observed repression of genes involved in cell envelope synthesis, cell
division, and replication, which was not as obvious in S9 cell internalized staphylococci.
Nevertheless, cell wall remodeling processes were concluded from gene expression data (this
study). Furthermore, staphylococci in neutrophils induced several toxins and adhesins (Voyich et
al. 2005), of which some were also observed after S9 cell internalization, e. g. hlgB, hlgC, lukD,
lukE; fnbB, coa, clfA (this study). The toxin gene induction appeared to be stronger in the
neutrophil than in the S9 study, although hla was induced after internalization in S9 cells and not
after phagocytosis by neutrophils. Anyway, the different staphylococcal strains, which were used
in the studies, contribute considerably to the characteristics of the gene expression signature
(Voyich et al. 2005). In the neutrophil phagocytosis dependent gene expression pattern,
differential expression of regulatory genes was reported (Voyich et al. 2005). These included
increased expression of vraRS, saeRS, and sarA. While induction or a trend of induction of saeRS
and vraRS was also visible in this study, sarA was not differentially expressed when staphylococci
were internalized in S9 cells. Furthermore, repression of agr or sigB was not observed in this
study, but in the study of Voyich and coworkers.
In total, after comparison of the results of Voyich and colleagues (2005), Garzoni and
colleagues (2007), and this study, it becomes clear that similarities in certain aspects do not
necessarily lead to completely consistent results. This probably depends to a large extent on the
combination of host cell and bacterial strain, but also on further experimental conditions which
varied strongly between the studies. Nevertheless, parts of the results were also found in other
studies. Since an admittedly slightly imprecise attempt to specify real internalization specific gene
expression that did not occur in control samples revealed virulence associated genes in the study
described in this thesis, the recorded gene expression pattern is probably physiologically
relevant. Therefore, the gene expression profiling turned out to be an important basic study
which will entail follow-up experiments like studies of bacterial mutants, comparison of different
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