genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
might be compensated at least in parts by others. Such compensation might contribute to the
non-distinguishable host reaction in the model described in this thesis. Additionally, the balance
between the different regulators might be different in in vivo settings than in the well-studied
in vitro conditions. Goerke et al. (2005) observed less SigB activity in vivo in a guinea pig model of
implant infection than in vitro under induced conditions like in the post-exponential growth
phase. But differences were also observed between in vitro and in vivo analyses in the sequential
order of gene expression pattern (Goerke et al. 2005).
Even more complicated, a transposon mutagenesis study identified 13 SigB-independently
transcribed genes whose mutation led to an increase in hla transcription and protease activity.
This effect had a different magnitude depending of the mutation-affected gene and could be
traced back to a gradual reduction of SigB activity after mutation. As these genes are part of
normal cellular metabolic pathways and do not exhibit DNA binding motifs the authors propose
them to be indirectly involved in the control of RsbU activity, preventing full signaling through
RsbU but not fully blocking the phosphatase activity. Such hypothesis assigns these metabolic
genes a function similar to that of the B. subtilis RsbRSTX regulation system sensing
environmental conditions, which is absent in S. aureus (Shaw et al. 2006).
In this context, the question arises whether activation of SigB really takes place in the specific
in vivo infection setting or whether there are differences depending on the model used, the site
of infection or other experimental factors influencing S. aureus during infection. Reduced in vivo
activity of SigB might also result in the similarity of host reaction to infection with S. aureus
RN1HG and its sigB mutant as described in this thesis. The question whether sigB and the SigB
regulon are really expressed in infection models strongly suggests the examination of sigB and
SigB-dependent marker genes by real-time qPCR in infected samples for further investigations on
the relevance of SigB in in vivo settings.
In summary, the results of this study do not provide any hints for differences in the
pathogenesis or pathomechanism of the S. aureus strains RN1HG and ΔsigB in the selected model
of i. v. infection in mice. If really existing, such differences might be transient and only apparent
at earlier time points. Effects of SigB might also be superimposed in in vivo infection by the
interlaced pattern of other regulators. There is also the possibility of missing activity of SigB
in vivo which could explain the similarity of host reaction to infection with S. aureus RN1HG and
its sigB mutant in the model used in this study. SigB might possess only to a lesser extent
characteristics attributed to virulence factors and might act in vivo more like a virulence
modulator and fine tune bacterial reactions. Assuming such function, the missing of detectable
differences in the host’s reaction to S. aureus RN1HG and its isogenic sigB mutant is explainable.
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