Elucidating functions of a specialised staphylococcal secretion system during infection

Staphylococcus aureus is a highly versatile bacterial pathogen responsible for serious infections in humans and animals. Hospital associated antibiotic resistant S. aureus infections ranging from skin infections to septic shock are responsible for increasing healthcare costs globally. The rise of new antibiotic resistant strains is major hurdle in effective treatment of S. aureus infections. Recurring episodes of skin and soft tissue infections are extremely common and difficult to eliminate.

While S. aureus usually acts from outside the eukaryotic cells, it can enter inside host cells and multiply inside, using it as a niche to evade antibiotics and host responses. We do not completely understand the proteins and pathways that S. aureus uses to survive in the hostile host cell environment. Intracellular bacteria may play an important role in chronic staphylococcal infections. However, precise mechanisms by which intracellular bacteria cause persistent or chronic infections remain unclear.

S. aureus exports several factors to the external environment during infection. It contains a specialised protein export system called Ess, which secretes proteins that are important for virulence of the bacterium and good vaccine candidates. This export system is also found in other important intracellular pathogens like the tuberculosis bacillus. While we know much about the TB systems, the functions of the staphylococcal apparatus are not known. Our recent research demonstrated that one of these Ess proteins, EsxA allow host cells infected with S. aureus to survive longer. We do not understand how these Ess proteins interfere with pathways inside cells or with our immune responses to this pathogen. The main goals of this project are to understand, at a molecular and cellular level, the biological role of the Ess proteins during persistent staphylococcal infections. Using genetic, cellular and immunological techniques we will determine how the bacterium uses the Esx proteins to block cell proteins and pathways. We will explore the mode of action of these proteins during chronic staphylococcal disease using in vivo models of disease.

Our studies will reveal novel pathways of host subversion during persistent staphylococcal infections and contribute to better understanding of the biology of the fascinating Ess systems.

The human pathogen Staphylococcus aureus is a leading cause of healthcare-associated infections. The advent of new antibiotic resistant strains and higher recurrence rates of staphylococcal infections have placed a significant cost burden on healthcare systems worldwide. Chronic infections have been associated with intracellular S. aureus, although intracellular mechanisms mediating bacterial persistence are poorly studied. Esat-6 secretion system (Ess), a specialised type VII secretion system of S. aureus, is required for staphylococcal virulence and persistence. Proteins secreted by the Ess, including EsxA and EsxB, are crucial virulence factors and putative vaccine candidates. The molecular mechanisms underlying Ess functions during persistent staphylococcal infection are currently unclear. Our recent work demonstrates that the Esx proteins play a role in intracellular staphylococcal infection by interfering with host cell death pathways. The goal of this project is to understand the mechanism of action of Ess during staphylococcal infection. Employing isogenic mutants, high-resolution microscopy and biochemical methods we will elucidate host cellular pathways that Ess proteins interfere with during intracellular infection. We will examine Ess-associated local immune and cellular responses in ex vivo and in vivo models of acute and chronic infection. Deciphering the biological functions of the Ess proteins will further our understanding of these intriguing bacterial systems and unravel novel staphylococcal mechanisms for intracellular survival and persistence.

Our research will reveal new aspects of the biology of S. aureus persistence and identify Ess proteins that may have potential as vaccine or drug targets. The proposed study has a wide range of beneficiaries:

1. The proposed research will highly benefit companies with interests in developing novel drugs or vaccines against S. aureus. Combination vaccines/ drugs against persistent disease are expected to have excellent outcomes in disease control. Data regarding mode of action of targets is crucial in the design of effective vaccines. This project will produce valuable information regarding novel proteins that mediate intracellular survival, and that have good potential as drug or vaccine targets. Impact of any commercially valuable data will be maximised by liaising with the Warwick Ventures group, a subsidiary of the University of Warwick that plays a key role in building the University's reputation for research excellence.

2. S. aureus cause a range of human infections in hospital and community settings. Staphylococcal infections have considerable economic impact on the healthcare system in the UK and in developed and developing parts of the world. We expect that in the long term, the data from this research will have an indirect impact on improving the management of staphylococcal infections in healthcare centres and hospitals.

3. Clinical and sub-clinical staphylococcal mastitis have a significant impact on health and welfare of cattle. An effective vaccine against S. aureus could also immensely benefit the dairy industry. Veterinary heath professionals who are involved in animal healthcare will be interested in the findings of our study.

4. Given the impact on the healthcare sector, the general public will benefit from the findings of our study. We will communicate the findings of our research through press releases, use of social media and through University events open to the general public to showcase our research.

6. The project will contribute significantly towards understanding the basic biology of bacterial protein export systems and host-staphylococcal interactions. This work will have impact on microbiologists interested in bacterial persistence, secretion, host cellular biology and host-microbe interactions. Findings from our study will be presented at general microbiology or cellular microbiology meetings that are attended by scientists from different disciplines, and published in high impact scientific journals with wide readership.

7. The staff employed (a qualified postdoctoral fellow) to perform this research will benefit by gaining excellent training in molecular and cellular microbiology. The fellow will receive formal training in the use of animals in biological research. The fellow will also benefit from the various training courses geared towards career and personal development available at University of Warwick.