Investigating bacterial virulence using an alternative host-pathogen model

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Virulence is defined as the ability of a bacterial pathogen to injure a host organism by producing factors that are essential for the pathogen to invade and multiply in the host. These factors must be identified and functionally characterized in order to understand the behaviour of bacterial pathogens. The long-term objective of my research program is to identify the molecular basis of the virulence factors produced by pathogenic bacteria affecting fish as well as ways to inhibit or modulate them to prevent or treat bacterial infections in fish farms. I use an approach that combines bacterial genomics with the use of the alternative host model Dictyostelium discoideum, a soil amoeba that is a natural predator of bacteria, to achieve this goal. D. discoideum is a particularly useful tool for studying the virulence of psychrophilic pathogenic bacteria, including those that affect fish such as salmonids. Over the next five years, my research program will continue to focus on the virulence of Aeromonas salmonicida. This bacterium is a fish pathogen that has a major negative impact on aquaculture in Canada. Using the amoeba host model, we have shown that this bacterium is sensitive to stressful conditions, which cause genomic modifications that affect major virulence factors, including the type three secretion system, leading to a loss of virulence. The genomic plasticity of A. salmonicida results from the presence of abundant mobile genetic elements in its genome. Using next generation sequencing and genotyping of multiple A. salmonicida strains, we have preliminary evidence of the presence of new uncharacterized virulence factors. Based on the hypothesis that important virulence factors of A. salmonicida are affected by the activity of mobile genetic elements, we propose that other virulence factors in this bacterium are also affected by the activity of mobile genetic elements and may be targets for the prevention or treatment of A. salmonicida infections. The identification and characterization of A. salmonicida virulence factors and the evaluation of their sensitivity to the direct effect of compounds isolated from natural products or through the action of mobile genetic elements stimulated by these compounds is a promising approach. My research program will thus focus on the following short-term objectives: (1) Identify new virulence factors of A. salmonicida by genetic screening using D. discoideum as a host and by comparative genomics. (2) Functionally characterize these new virulence factors using various approaches, including challenges in fish. This part of the study will also involve the development of new genetic tools to efficiently manipulate A. salmonicida genes. (3) Identify aquaculture-compatible compounds that directly inhibit virulence factors or disrupt their genes by stimulating mobile genetic elements. The effect of these compounds on A. salmonicida virulence will be screened using D. discoideum as a host. The most promising compounds will then be tested on fish. My research program will provide a better understanding of the virulence factors of A. salmonicida. Based on our on-going effort to understand the pathogenic basis of A. salmonicida and the objectives set out in this proposal, we are confident that we will be able to modulate and inhibit the virulence of this bacterium by taking advantage of its genomic plasticity. This research is crucial for the development of novel anti-bacterial drugs that target virulence factors and that can be used as alternatives to antibiotics not only in the veterinary field but also for human health.