New ways to control Shigella infection in the zebrafish model

Shigella flexneri is an invasive bacterial pathogen and inflammatory paradigm that has led to key discoveries in innate immunity. The transparent zebrafish (Danio rerio) larva is highly amenable to in vivo microscopy and has emerged as a valuable model for the study of host defence against bacterial infection. In this thesis, I use the zebrafish to explore the innate immune response to Shigella infection, and illuminate new ways to control bacterial infection in vivo. Septins are an evolutionarily conserved family of cytoskeletal proteins with diverse biological functions. Very recent work has shown a key role for components of the cytoskeleton in inflammation control, yet a role for septins was unknown. In Chapter 3, I develop a zebrafish hindbrain model of Shigella infection and reveal a new role for septins in the restriction of inflammation crucial for host defence. The global rise in antibiotic resistance amongst human pathogens has prompted an urgent search for novel antimicrobials. Bdellovibrio bacteriovorus are predatory bacteria that invade and kill Gram-negative species. Bacterial predation has been studied extensively in vitro but little is known of its efficacy in vivo. In Chapter 4, I use the Shigella-zebrafish infection model developed in Chapter 3 to show how immune cells and Bdellovibrio can work together to cure infection from multidrug resistant Shigella. Septins were discovered for their essential roles in cell division. S. flexneri is controlled by neutrophils in vivo, and emergency granulopoiesis is a haematopoietic program of neutrophil production used to counteract immune cell exhaustion during infection. In Chapter 5, I use Shigella infection of zebrafish to reveal a cell autonomous role for septins in haematopoietic stem cell-driven emergency granulopoiesis and in enhancing host immunity. Overall, the findings in this thesis have important implications for the treatment of inflammatory, infectious and haematological disease. As a result of these data, manipulation of the septin cytoskeleton represents a novel strategy by which to control inflammation. Furthermore, results obtained using Bdellovibrio are promising for the future consideration of living therapies to treat bacterial infection. Finally, the host cell division machinery can be used to modulate the haematopoietic response to infection and boost host defence. Collectively, results obtained using our Shigella-zebrafish infection model are highly translatable to higher vertebrates, including humans.