Investigation of the role of the Type VII secretion systems in Staphylococcus aureus-macrophage interactions

Staphylococcus aureus, a bacterium often found on our skin and nares, is one of the major causes for life threatening infections such as pneumonia and endocarditis. This pathogen is frequently associated with healthcare systems and hospital-associated S. aureus infections have been responsible for increasing healthcare costs globally. The rise of new antibiotic resistant strains has been a major hurdle in effective treatment of S. aureus infections with the available drug therapies. A vaccine against resistant S. aureus infections could provide an effective solution to antimicrobial resistance.

During infection, S. aureus is known to closely interact with our cells. In addition to acting from outside cells, this bacterium can enter inside our cells using it as a niche to multiply. Immune cells called macrophages, which form the first line of defence, are important in pathogen clearance. However, studies have indicated that these cells are not always effective in killing S. aureus. S. aureus is able to survive and multiply within macrophages, which then inadvertently aid in transporting bacteria to other body sites. We do not currently have a good understanding of the bacterial proteins and pathways that this pathogen uses to manipulate these immune cells.

S. aureus exports several proteins to the external environment during infection. It has a specialised protein export system called the type VII secretion system (T7SS), which secretes proteins that are important for virulence of the bacterium. These proteins have also shown good vaccine potential. The precise biological functions of the T7SS proteins during infection are not known. Our recent research demonstrated that the T7SS proteins can control macrophage death. But, we do not understand how these proteins interfere with pathways inside cells or with our immune responses to this pathogen. The main goals of this project are to understand how the T7SS proteins interfere in host signalling pathways and how this impacts staphylococcal infection outcomes. We will use a combination of cellular, biochemical and high throughput assays to study proteins and pathways that interact with the T7SS. We will also explore the mode of action of these proteins during lung infections employing a laboratory model of staphylococcal pneumonia.

Our studies will reveal novel pathways of host subversion and new insight into the fascinating T7 export systems. Importantly we believe these studies would inform the development of these proteins for therapeutic and prophylactic use.

The human pathogen Staphylococcus aureus is a leading cause of serious healthcare-associated infections like pneumonia. S. aureus elaborates a range of virulence factors including a specialised type VII secretion system (T7SS). Proteins secreted by the T7SS are crucial for bacterial virulence, induction of immune responses and are putative vaccine candidates. The molecular mechanisms underlying T7SS function during 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 in macrophages, key immune cells during infection. The overall goal of this project is to understand the mechanism of action of T7SS, specifically focusing on how it controls macrophage cell death and local immune responses to infection. Employing isogenic mutants, high-resolution microscopy and biochemical methods we will study how the T7SS components activate cell death pathways during intracellular infection. High throughput methods will be used to identify T7SS-host protein interactions and host pathways that are activated. We will examine T7SS-mediated immune cell recruitment in ex vivo and in vivo models of staphylococcal pneumonia. Deciphering the biological functions of the T7SS proteins will further our understanding of these intriguing bacterial systems, reveal new mechanisms of host manipulation by pathogens and provide valuable information to support therapeutic and prophylactic development of these proteins.