genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
significant decrease was apparent after infection with the sigB mutant compared to the parental
strain (Lorenz et al. 2008). Others have published that sigB mutations have no influence on cfu
counts. This emphasizes the important influence of other experimental parameters like infection
dose, infection route, site of infection or its manifestation, analysis time point after infection, the
host factors – at least in parts determined by the host genome – acting on the bacterium, and the
genetic background of the pathogen.
Repair of rsbU or sigB overexpression in the genetic background of BB255, a rsbU mutated
S. aureus strain, leads to an increase in transcription of clfA, mediated via a SigB-promoter in the
clfA gene, and an increase in transcription of fnbA which is indirectly caused by SigB and probably
mediated via a positive effect of SigB on sarA and subsequently of SarA on the fnbA transcription.
The sigB overexpression mutant features an increased attachment to fibrinogen and fibronectin
coated surfaces and to platelet-fibrin clots, which imitate the environment in injured
endothelium of blood vessels. When the same mutant is applied in an infection setup focusing on
adherence by inducing catheter-associated aortic vegetation in rat, a transient effect of higher
bacterial density in early stages of infection was observed. After long-term infection, the
advantage of the early phase was counterbalanced in comparison to sigB deficient strains. The
authors argue that S. aureus is still able to produce sufficient amounts of adhesins leading to
aortic vegetation even without a functional sigB (Entenza et al. 2005).
In a murine sepsis and arthritis model, the SigB positive strain SH1000 (rsbU repaired) caused
more severe infection than strain 8325-4 which is phenotypically SigB − (11 bp rsbU deletion) as
illustrated by higher mortality, more pronounced weight loss and higher serum levels of IL-6 as
indicator of inflammation. Additionally, at day 14 after i. v. infection higher arthritis frequency
and severity were observed after infection with SH1000. In an attempt to elucidate whether the
effect originated from increased elimination of the SigB negative strain or a decreased virulence
in situ the authors infected mice intra-articularly. In this experiment no difference in the ability to
cause inflammation was recognized between infections with the two strains. This led to the
conclusion that SigB-influenced steps before arthritical joint involvement contributed to the
differences between the strains after i. v. infection, possibly including SigB regulated cell surface
adhesins as crucial virulence factors (Jonsson et al. 2004).
In summary, the impact of SigB in vivo is still disputed, and results in literature references are
partly inconsistent. A hypothesis reconfirmed by several authors is that the effect of SigB in
in vivo infection models possibly only manifests in early phases of the infection and only impacts
processes in a transient manner.
Hence, it can be speculated that in the model described in this thesis the analyzed time point
might have been too late for detection of differences in the host reaction.
Furthermore, the effects of SigB are part of a complex and interfering regulation mechanism
including several other regulatory molecules like RNAIII from the agr locus or SarA. SigB
dependent transcription of sar locus was demonstrated (Ziebandt et al. 2004). The SarA protein is
a transcriptional regulator, which represses (e. g. spa) or activates (e. g. fnbA) gene expression
directly. It also activates expression of the agr system and therefore influences indirectly the
expression of genes responding to RNAIII (Chien et al. 1999). SigB itself has a negative effect on
the amount of RNAIII (Horsburgh et al. 2002b). Several genes or their promoter regions possess
binding sites for more than one transcriptional regulator or sigma factor, e. g. the sae locus coded
two-component system also influences SigB-regulated target genes (Goerke et al. 2005).
Therefore, the transcriptional pattern of the regulators overlap, and the mutation of one of them
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