The Staphylococcus aureus secretome - TI Pharma

Introduction and scope of this thesis
Some of the staphylococcal virulence factors are proteins that are displayed at the cell wall
surface. In S. aureus some of these surface proteins are linked to the cell wall by so-called
sortases. In Chapter 6, the exoproteomes of S. aureus and S. epidermidis srtA mutants were
investigated and compared to the respective parental strains. Several SrtA substrates were
identified, and their final subcellular localization was found to be altered in the srtA mutants.
Among these identified proteins were the S. aureus surface protein G (SasG) and the
accumulation associated protein (Aap) from S. epidermidis. Biofilm formation was affected in
the srtA mutants. Complementation studies were performed with SrtA from S. aureus and S.
epidermidis as well as with SrtC from S. epidermidis and YhcS from B. subtilis. Only
complementation with S. aureus or S. epidermidis SrtA resulted in restoration of the parental
phenotype. In contrast, SrtC of S. epidermidis only partially seems to restore the parental
phenotype, and YhcS of B. subtilis was not able to complement for the loss of SrtA at all.
In Chapter 7, the susceptibility of bacteria towards the extremely stable and broad-spectrum
lantibiotic sublancin 168 was investigated. S. aureus is one of several important pathogens
that is susceptible towards sublancin 168. Growth inhibition and competition assays on plates
and in liquid cultures revealed that the addition of NaCl lowered the sublancin 168
susceptibility of S. aureus and B. subtilis. In addition, it was shown that the presence of the
large mechanosensitive channel of conductance MscL is important for the susceptibility of
these bacteria. Taken together, the results demonstrate that MscL is a critical and specific
determinant in bacterial sublancin 168 susceptibility that may either serve as a direct target for
this lantibiotic, or as a gate of entry to the cytoplasm.
Chapter 8 describes the exoproteome of the commercially interesting organism B.
licheniformis. With the annotated genome sequence of B. licheniformis DSM 13, a secretome
prediction analysis was performed. A total of 296 proteins were predicted to contain an Nterminal
signal peptide directing most of them into the Sec pathway. From analysis of the
extracellular proteomes of B. licheniformis it was concluded that higher amounts of protein
are secreted when cell are grown in a complex medium compared to cell grown in a minimal
medium. In addition, limitation of phosphate, carbon and nitrogen sources resulted in the
secretion of specific proteins that may be involved in counteracting the imposed starvation
conditions.
Finally, in Chapter 9 the results described in this thesis are discussed and ideas for future
research are presented.
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