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Investigating functions of the staphylococcal type VII secretion systems

Primary Supervisor: Dr Meera Unnikrishnan, WMS

Secondary supervisor: Professor David Roper

PhD project title: Investigating functions of the staphylococcal type VII secretion systems

University of Registration: University of Warwick

Project outline:

Staphylococcus aureus, a highly versatile bacterial pathogen, is a leading cause of infections in animals and humans. It causes a range of serious conditions in humans including pneumonia and endocarditis, and is major cause of hospital-associated infections. The high rates of recurrent infection and a rise in antibiotic resistance has made staphylococcal infection harder to treat and has resulted in increased healthcare associated costs. In animals, it causes mastitis, a severe infection of the mammary glands in dairy cows. S. aureus is responsible for 10%-20% of the clinical and subclinical mastitis cases studied. Decreased milk production and increased expenditure due to mastitis have resulted in huge financial losses in the dairy industry worldwide. Subclinical mastitis is a chronic condition common in dairy herds with no visible clinical symptoms. Persistent sub clinical infections are very hard to eradicate.

  1. aureus expresses an arsenal of virulence factors, most of which are secreted through conventional secretion systems. S. aureus also encodes a specialised type VII Esat-6 secretion system (Ess or T7SS), a system that was originally described in the intracellular pathogen, Mycobacterium tuberculosis and demonstrated to be crucial for intracellular survival within macrophages (1). The staphylococcal T7SS encodes virulence factors required for staphylococcal virulence and persistence in murine staphylococcal infection models. Our recent work demonstrated that Ess substrates interferes with host cell death pathways during intracellular staphylococcal infection, as well as modulating cytokine responses. The molecular mechanisms underlying Ess-mediated modulation of cellular processes and the precise role of Ess during persistent staphylococcal infection is currently unclear.

Recent studies from our group have also shown a role for the T7SS proteins protection from antimicrobial agents including fatty acids, and implicate a role for this system in bacterial membrane homeostasis (2). While some T7SS proteins have already been considered as vaccine targets, these proteins may also have good potential as drug targets.

The goal of this project is to understand mechanisms of action of the T7SS during S. aureus infection. Roles of individual T7SS components will be investigated employing isogenic mutants and in vitro cell infection assays. T7SS-associated local immune and cellular responses will be studied in murine models of acute and chronic infection. The project will span across cellular microbiology, immunology and biochemistry and will involve a range of techniques including infection assays, confocal microscopy, live imaging and immunological assays.


  1. Unnikrishnan, M*, Constantinidou, C., Palmer, C and Pallen, M (2017) The enigmatic Esx proteins: looking beyond mycobacteria Trends Microbiol, 25(3) 192-204
  2. Korea, C.G., Balsamo, G., Pezzicoli, A., Merakou, C., Tavarini, S., Bagnoli, F., Serruto, D., and Unnikrishnan, M. (2014) Staphylococcal Esx proteins modulate apoptosis and release of intracellular Staphylococcus aureus during Infection in epithelial cells.Infect. Immun. 82(10):4144

BBSRC Strategic Research Priority: Understanding the Rules of Life: Immunology & Microbiology

      Techniques that will be undertaken during the project:

      • Handling and culturing CL2 pathogens
      • Cloning and generation of mutants
      • Growth assays
      • Inhibitor screening
      • Fluorescent confocal and widefield microscopy
      • Live imaging
      • Immunoblotting
      • ELISAs
      • mammalian tissue culture
      • in vitro infection assays

      Contact: Dr Meera Unnikrishnan, University of Warwick