genomewide characterization of host-pathogen interactions by ...

Maren Depke
Introduction
shock are also effective in activating SigB in S. aureus. The entry into stationary growth phase
additionally leads to the activation of the SigB regulon. Only a minor fraction of B. subtilis SigB
dependent genes is found as orthologue in S. aureus and vice versa. In an approach to
functionally characterize the SigB regulon of S. aureus, physiological aspects like cell envelope
composition, membrane transport processes, and intermediary metabolism emerged as affected
(Pané-Farré et al. 2006).
The SigB regulon contains several virulence associated genes like coa, fnbA, ssaA, clfA, hla,
hlgABC, lip, nuc, and sak. In a general view, a pattern of induction of cell surface virulence factors
and a repression of exoproteins and toxins by SigB is recognized. Therefore, SigB effects are
inverse to the effects of the active agr system via RNAIII (Bischoff et al. 2004).
Taken together, S. aureus exhibits a complex control of its virulence factors that includes
several interlaced pathways of central regulators like agr, sarA, and sigB which are interesting
targets for knock-out mutants to be tested in infection models.
Increasing Importance and Danger of S. aureus Infections
S. aureus is one of the main causes of hospital-and community-acquired infections, and its
prevalence and antibiotic resistance have been studied intensively (Diekema et al. 2001, Goto et
al. 2009).
S. aureus as pathogen causing infection could be treated for a long time with classical
antibiotics. But the pathogen has a potent ability to develop or acquire antibiotic resistances.
With today’s increasing cases of antibiotic-resistant staphylococcal strains the question whether
the era of untreatable infections has arrived is raised. This accentuated question emphasizes the
recently enhanced importance and danger of S. aureus and complementarily, the imparative of
strict infection control and the importance of discovering new antibiotics and vaccination targets
and developing these agents for medical purposes (Livermore 2009).
Staphylococcal strains which are not only resistant to long- and often-used antibiotics, but
also to those kept as reserve emerged and increase in prevalence in many countries (Livermore
2000, Fig. I.5). This took place for methicillin (methicillin-resistant S. aureus – MRSA; MRSA might
also be used in the context of multi-resistant S. aureus) since the 1960s, only some years after
start of clinical application, or more recently for vancomycin (vancomycin-resistant
S. aureus − VRSA) after S. aureus strains have obtained the vanA operon from an Enterococcus
spp. transposon on a conjugative plasmid (Péricon/Courvalin 2009). The Staphylococcal Cassette
Chromosome mec (SCCmec), a genetic region which is situated on a mobile genomic island and is
thought to originate from Staphylococcus sciuri (Wu et al. 2001), is responsible for resistance to
methicillin and all other ß-lactam antibiotics. This region exists in form of several different types
(I-VIII) and can be used along with protein A typing (spa), multilocus sequence typing (MLST), and
pulse-field gel electrophoresis (PFGE) to distinguish different staphylococcal lineages on a
molecular level (Deurenberg/Stobberingh 2008, Zhang et al. 2009). Staphylococcal resistance to
methicillin is based on different mechanisms. The main form is the expression of PBP2a, a nonmethicillin-sensitive
variant of the sensitive original penicillin-binding proteins (PBPs). PBPs, of
which S. aureus owns four types PBP1-4, have essential function in cell wall biosynthesis, where
they are responsible for cross-linking of peptide chains in the peptidoglycan and additionally
stretch the glycan part via their transglycosylase activity. Methicillin inhibits PBP’s peptide cross-
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