genomewide characterization of host-pathogen interactions by ...

Maren Depke
Discussion and Conclusions
mutants. Thus, especially saeRS as regulatory system, but also efb and SA1000 (SAOUHSC_01110)
contribute to adhesion and invasion (Liang et al. 2006). Strikingly, these genes were observed to
be induced (saeR, efb) or in trend induced (saeS; SAOUHSC_01110; data not shown for
SAOUHSC_01110) in internalized staphylococci in the study described in this thesis. Furthermore,
also coa, fnbB, hla, and hlgC, determined as SaeRS-dependent by Liang and colleagues, were
induced in internalized staphylococci in this study. The final conclusion of Liang et al. stated that
“activation of the SaeRS system is required for S. aureus to adhere to and invade epithelial cells”.
Supportingly, such activation was also concluded in this study.
Nevertheless, the activation SaeRS and induction of saeRS might depend on further
experimental aspects which are not defined yet. Infection and gene expression studies by Garzoni
and coworkers did not reveal induction but repression of saeR and saeS in S. aureus 6850 upon
internalization in human lung epithelial A549 cells (Garzoni et al. 2007). Contrarily, upon
phagocytosis by human polymorphonuclear leukocytes the induction of saeR and saeS was
observed in different S. aureus strains (Voyich et al. 2005).
While adherence of sae mutant and wild type to endothelial cells did not differ, the mutant
was less invasive than the wild type (Steinhuber et al. 2003). Further affirmation of the
participation of saeS in infection models was derived from mutagenesis studies in combination
with murine systemic infection. Here, non-functional saeS led to attenuated virulence (Benton et
al. 2004). A different bacterial strain background led to similar observations of reduced virulence
in mice (Rampone et al. 1996). Also Goerke et al. identified SaeRS as important regulator which is
active in vivo, although the deletion mutant did not yield different bacterial densities compared
to the wild type in a device-related infection model (Goerke et al. 2005).
Besides the already mentioned saeR and saeS, three other regulators were observed with
differential expression in internalized staphylococci (sarT, sarU, rot). Since sarT and sarU are less
well characterized and since all these three genes were repressed, the effect of this differential
expression cannot easily be rated in the setting of the in vitro infection model.
Fittingly to the concluded activity of the SaeRS system in internalized staphylococci,
membrane-bound adhesins, which are partly known to be regulated by SaeRS (Liang et al. 2006),
were induced (fnbA, fnbB, clfA, clfB).
Fibronectin binding proteins (FnBPs) are – together with other surface proteins – important
mediators of bacterial adhesion to host cells. They exhibit an even more central position for
internalization processes. FnBPs were partly required for adhesion and mainly required for
internalization into the bovine mammary gland epithelial cell line, MAC-T (Dziewanowska et al.
1999). The mechanism includes binding of FnBP to β 1 integrins via a bridge formed by fibronectin,
but also direct binding of FnBP to host membrane-located Hsp60 (Dziewanowska et al. 2000).
Anyway, another study demonstrated variability of fnb genes between different clinical isolates
as well as variation in the ability to adhere to fibronectin by the different isolates. Strains which
were derived from orthopaedic implant-associated infection showed higher adherence than the
other isolates. Community-acquired invasive disease isolates harbored more often two fnb genes
than carriage isolates, but the number of fnb genes correlated only to a low extent with the
adhesion capacity (Peacock et al. 2000).
Fibronectin-binding protein deficiency reduced the colonization of heart tissue in a rat
endocarditis model indicating a role of FnBP in heart valve infection. Other tissue samples did not
show the same effect (Kuypers/Proctor 1989). In order to avoid compensation of a deleted gene
by other, functionally redundant staphylococcal genes, an overexpression study of clfA and fnbA
199