genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
in Lactococcus lactis, a bacterium with low pathogenic potential, was performed (Que et al.
2001). Possession of ClfA or FnbA rendered L. lactis as adherent to fibrinogen or fibronectin as
the gene-donor S. aureus strain. Furthermore, overexpressing L. lactis strains were able to cause
endocarditis in a rat model with a similar minimal infection dose as S. aureus. The authors stated
that this overexpression experiment proved the involvement of clfA and fnbA in endocarditis with
higher confidence than deletion experiments (Que et al. 2001).
Thus, fnb genes have proven to be relevant for adhesion, invasion and in vivo infection
situations. Surprisingly, the induction of adhesins was observed after internalization of S. aureus
RN1HG into S9 cells (this study). It can be speculated that this induction took place in advance to
react to a possible re-liberation from the host cell e. g. after host cell death.
In internalized staphylococci, also soluble adhesins were induced (eap, coa, vwb, emp, efb).
These are also associated with other functions like immune-evasion. Eap, whose expression is
essentially mediated by sae (Harraghy et al. 2005), is one of the examples for a staphylococcal
protein which is related to a variety of functions (Harraghy et al. 2003). Eap was reported for
example to mediate adhesion to cultured fibroblasts (Hussain et al. 2002), to enhance
internalization into human fetal lung fibroblasts and HACAT keratinocytes (Haggar et al. 2003), to
impair wound healing via inhibition of angiogenesis (Athanasopoulos et al. 2006), to block Ras
activation and signal transduction cascade which leads to anti-angiogenesis (Sobke et al. 2006),
to induce pro-inflammatory IL-6 and TNF-α in human peripheral blood mononuclear cells (Scriba
et al. 2008), but also to act anti-inflammatorily by inhibition of leukocyte recruitment via
blockade of ICAM1 and hence lead to immune evasion (Chavakis et al. 2002, Haggar et al. 2004).
Thus, it was not surprising to find induced expression also after internalization into S9 cells (this
study). Further immune-evasion related genes were increased in S9-internalized S. aureus
RN1HG, e. g. chp, whose gene product CHIPS, chemotaxis inhibitory protein of staphylococci, is
known to block the receptor for the chemotactic complement fragment C5a and for bacterialderived
formylated peptides (de Haas et al. 2004, Murdoch/Finn 2000). Also induced efb, coding
for extracellular fibrinogen-binding protein Efb, interferes with complement action, i. e.
opsonization, by inhibition of C3b-binding to bacterial surfaces (Lee et al. 2004a, 2004b). Thus,
internalized staphylococci induce certain genes which would benefit their survival in an in vivo
situation.
In aerobically living cells, reactive oxygen intermediates like hydroxyl radicals (OH•), hydroxide
(OH – ), superoxide anion (O – 2 ), or hydrogen peroxide (H 2 O 2 ) occur in normal metabolism and may
damage cellular structures like DNA (Fridovich 1978, Imlay/Linn 1988). Phagocytes even produce
high amounts of reactive oxygen intermediates during respiratory burst, e. g. superoxide anion by
their NADPH-oxidase, as part of their defense and killing strategy (Robinson 2009). S. aureus
owns two genes coding for superoxide dismutases: sodA and sodM (Clements et al. 1999,
Valderas/Hart 2001). The enzyme participates in the detoxification of reactive oxygen
intermediates by catalyzing the reaction of two superoxide anion molecules to oxygen and
hydrogen peroxide, of which two molecules are consequently decomposed to oxygen and water
by catalase. The complete enzyme constitutes of 2 homo- or heterodimeric subunits. The gene
sodM is characteristic for S. aureus since coagulase-negative staphylococci like S. epidermidis do
not own this gene or a homologue (Valderas et al. 2002).
In the presence of manganese, the internal (methyl viologen) and external (xanthine/xanthine
oxidase) generation of O –
2 in S. aureus led to increased expression of sodA and sodM,
respectively, but not without the addition of manganese (Karavolos et al. 2003). The observation
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