Interaction of Staphylococcus aureus and humans: Iron regulated surface proteins and a novel host defence mechanism

Staphylococcus aureus is an important human pathogen, particularly in hospitals where it kills many people each year. The problem is compounded by the spread of antibiotic resistance and we know the organism best as MRSA (methiciliin resistant S. aureus). We have identified a number of S. aureus components that are being developed as a vaccine to prevent disease and spread of the bug. We have also found that one of these bug proteins protects it against a number of the mechanisms we humans make to defend ourselves. In doing so we have found a new way in which humans are able to turn off the toxins made by S. aureus. We will find out how this mechanism works and how the bug is able to defend itself in this battle. The research will define how the bugs and we have evolved to interact with each other and give novel potential avenues to sway the outcome in our favour.

Staphylococcus aureus is a major human pathogen of increasing importance due to the spread of antibiotic resistance. It is relatively ubiquitous in the human environment due to nasal carriage. At present there is no vaccine available. As part of a large study we have identified S. aureus antigens expressed during human infection. In particular we have found an iron regulated surface protein (IsdA), required for nasal colonisation. Vaccination against IsdA protects against nasal carriage and it (alongside other proteins) is currently being developed as a potential human vaccine. IsdA is an adhesin, required for binding S. aureus to human nasal squamous cells. IsdA also makes S. aureus more hydrophilic and is involved in resistance to human innate defence mechanisms, including defensins and skin fatty acids. Our analysis has identified a novel innate defence mechanism whereby human skin fatty acid is able to inhibit virulence determinant production and IsdA is required for resistance to this. IsdA is a member of a 4-protein family, the others with ill-defined roles.

The aims of the project are to characterise the roles of the 4 wall-associated Isd proteins of S. aureus, in particular IsdA, in host:pathogen interaction, cellular physiology and the action of antibiotics. Also the molecular basis and extent of the novel innate defence properties of skin fatty acids will be determined. The research objectives will determine the roles of the Isd proteins by creation of single and multiple mutants and the heterologous expression of proteins. The genetic constructs will be used to define ligand-binding capacity, resistance to host defences, role in iron acquisition and interaction with animal and human hosts. How IsdA is able to carry out its various functions in host defence resistance will be analysed by expression of individual domains (and sub-domains) and functional analysis. The mechanism of action of human fatty acids on bacterial survival and virulence determinant production will be determined using a combination of physiological, biochemical and genetic approaches. Finally the involvement of skin fatty acids and IsdA in the sensitivity of S. aureus to antibiotics will be elucidated. The ability of fatty acids to potentiate the activity of specific antibiotics will also be tested.

The project will define a novel mode of human innate defence, which might be important against a wider range of pathogens. Also the role of bacterial components in host resistance will be elucidated, as important facets of host:pathogen interaction.