Transmission and biofilm formation by pathogenic Salmonella strains

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Salmonella are a group of pathogenic bacteria that cause infections in mammals (including humans), birds and reptiles. Despite studying these pathogens for over 100 years, scientists still do not understand how the dynamics of Salmonella transmission occur and the mechanisms of survival and persistence remain elusive. My research program is focused on Salmonella biofilm formation, virulence, transmission and vaccine development. Biofilms are multicellular aggregative bacterial communities that may form on living and non-living surfaces; these structures are highly resistant to disinfectants, antibiotics, and environmental and metabolic stress. In this proposal, we will examine biofilm formation in Salmonella serovar Typhimurium, which is one of the most common groups of strains circulating in the world. In a previous NSERC discovery grant we discovered that when S. Typhimurium forms biofilms, the population of cells differentiate into resistant, biofilm cells and virulent, single cells. We hypothesize that this strategy, called `phenotype switching' provides a transmission advantage for S. Typhimurium strains when they pass into the environment. The two cell types of the population each provide specific advantages in different contexts, but together would ensure that transmission through the environment occurs efficiently. We will investigate the role of biofilms in S. Typhimurium transmission via contaminated water and the mouse model of infection, and use a technique called transposon sequencing (Tn-seq) to identify genes involved in long-term water survival and genes that are vital at key points in the transmission process. We will develop new genetic tools to test how phenotype switching is regulated, using existing fluorescently-tagged S. Typhimurium reporter strains (biofilm-GFP; virulent single cells-mCherry) and trying to build a dual-reporter strain so we can examine genetic regulation between the two cell types in a more detailed way. The experiments will involve extensive in vitro testing, including high-resolution, single-cell microscopy and microfluidics. Finally, we will investigate the host response to the specialized cell types of S. Typhimurium by performing cytokine profiling. This will aim to determine similarities and differences and how that relates to proteins and polysaccharides on the Salmonella cell surface. Biofilms are thought to be the predominant form of bacterial life and the role of different genes in transmission is an under explored area in Salmonella research. I want to understand the mechanisms that control phenotype switching, for if we can understand control and identify the signals that regulate this process, then we can design strategies to prevent it. Finally, the similarities with biofilm formation and the importance of transmission for the evolutionary success of pathogens means that our results will have a wide relevance and impact across the bacterial kingdom.