The Sae two-component system of Staphylococcus aureus: sensing mechanism and impact on bacteria-phagocyte interaction

In the human pathogen Staphylococcus aureus (S. aureus), two component systems (TCS) enable the bacteria to respond to different environmental stimuli. The core genome of S. aureus encodes 16 TCSs of which only walKR is essential. Recent studies showed that all TCSs can act functionally autonomous allowing the bacteria to sense, respond and adapt to environmental changes. For most TCS the mechanism leading to activation remains unclear. The SaePQRS System regulates the expression of various secreted and cell bound virulence factors which were shown to be important for the interaction with our immune system. It was shown that the Sae system is activated by phagocytosis related signals, specifically human neutrophil peptides (HNP1-3). Promotor activities and target genes are well studied whereas the mechanism of signal sensing is unknown. Here I show that the D-alanylation of wall teichoic acids (WTA) is important for HNP1-3 sensing. Studies on the regulation of dltABCD as well as a gene complementation approach could not explain strain specificity. Thereby I hypothesize, that strain differences in the regulation of teichoic acids or their alanylation is the reason for strain specificity and that this distinct composition is not imitable in an experimental setup. Furthermore, I could confirm sodium hypochlorite as another activator of the Sae-System which interestingly is not a strain specific activator. S. aureus is able to survive and even escape from professional phagocytes. Whereas lysis of neutrophils by S. aureus is well studied, factor(s) required for the escape from human macrophages are less clear. I could show that the Agr-regulated PSMs are responsible for the escape of S. aureus from the phagosome into the cytoplasm whereas the Sae-regulated two component toxins LukAB and PVL are mainly involved in cell death and subsequent escape of the bacteria from within the cells. Complete escape could only be enabled with the combined action of Agr- and Sae-regulated virulence factors. Altogether, my results shed light on the sensing mechanism of the important virulence regulator Sae. It is the first study, showing that WTA is involved in the signal sensing of a two-component system. Furthermore, I could show that the Sae-system plays a major role in the interaction with human macrophages after phagocytosis. Concerning the role of the intracellular lifestyle of S. aureus for bacterial dissemination in the human body, a deeper understanding of post-phagocytosis events is of great interest