genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
KIDNEY GENE EXPRESSION PATTERN IN AN
IN VIVO INFECTION MODEL
The SigB regulon contains several virulence associated genes, and expression of this regulon in
general promotes adhesion and reduces production of extracellular toxins (Bischoff et al. 2004).
While regulation of SigB activity has already been studied in vitro (Senn et al. 2005), its impact on
in vivo physiology, e. g. in infection models, is still matter of debate. The aim of this study was the
analysis of the relevance of SigB-dependent gene expression regulation for the adaptation of the
host transcriptome in murine infection.
In an i. v. murine infection model the two Staphylococcus aureus strains RN1HG and its
isogenic sigB mutant were used to infect mice with comparable infection doses. In mouse kidney,
the infection resulted in similar infection rates of approximately 1.0E+06 cfu/10 mg of tissue. The
role of SigB has already been analyzed in different in vivo infection models. A general observation
of these studies was that sigB deletion mutants and their parental strain can accomplish
comparable bacterial loads. In the clinical isolate WCUH29 the allelic replacement of sigB did not
lead to differing bacterial load in organs or wounds after testing original strain and isogenic
mutant in different types of infection models (Nicholas et al. 1999). Also Chan et al. (1998)
observed similar numbers of cfu in a skin abcess model, but the results might have been
compromised by the fact that the group used a sigB deletion in the 8325-4 background which
itself harbors an inactivating mutation in rsbU, a gene for a SigB regulatory phosphatase
(Kullik/Giachino 1997). Horsburgh et al. published in 2002 the construction of S. aureus SH1000, a
variant of strain 8325-4, in which the rsbU gene had been repaired. SH1000 and 8325-4 resulted
in the recoverage of similar numbers of cfu in a murine subcutaneous skin abcess model
(Horsburgh et al. 2002b). Contrarily, another study reported a significantly different bacterial
burden in kidney after i. v. infection with S. aureus 8325-4 (RsbU − ), SH1000 (RsbU + ), and a sigB
knockout mutant of SH1000 (SigB − ). In this setting, the repair of rsbU leads to an increased cfu
count 14 days after inoculation (Jonsson et al. 2004). In the study described in this thesis, almost
equal mean values of infection rate were observed for S. aureus RN1HG and its isogenic sigB
mutant, a further confirmation of literature data indicating similar virulence of sigB deficient and
positive S. aureus strains.
In order to gain a much more detailed view of the host reaction to both strains, kidney gene
expression profiles were compared using Affymetrix GeneChip arrays. In a total number of 19
infected samples, highly similar gene expression profiles were recorded 4 or 5 days after infection
with RN1HG or its sigB derivative. Even with this sophisticated analysis no difference in the
expression pattern in murine kidney after infection with the two strains was observed, indicating
that SigB indeed does not influence the host response.
However, the comparison to sham-infected samples revealed a rather strong reaction of the
murine host to infection, bacterial accumulation and proliferation in the kidney. Several immune
response pathways were induced on transcriptional level indicating a vivid activation of the
immune response. This applies first to pathways of the innate immune system: Pattern
recognition receptors (PRR) like toll-like receptors (TLR) and complement system components
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