Investigating host-induced lipopolysaccharide changes in the legume symbiont Sinorhizobium meliloti

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Bacteria can have both beneficial and negative impacts upon life. For example, the interaction between the soil bacterium, Sinorhizobium meliloti, and leguminous plants is beneficial since the bacteria convert nitrogen from the atmosphere into a form which can be utilised by plants. In return, the bacteria obtain nutrients from the plant. The outcome is a symbiosis where the bacteria survive long-term within the plant and promote plant growth. I am interested in understanding the factors, which enable the bacteria to survive long-term within plant cells. This is important since leguminous plants are major food sources and, since the nitrogen source produced by the bacteria is eventually released into the soil, this process alleviates the need to add costly nitrogen fertilisers to the soil. Understanding how processes work, is the first step towards improving processes. Interestingly, the soil bacterium is related to another bacterium, Brucella abortus, which causes abortions in cattle and results in a severe infection in humans known as Brucellosis. The factors important for Brucella to cause disease are poorly understood, but like the soil bacteria within plant cells, Brucella can survive long-term within animal and human cells. Cattle can be vaccinated to prevent Brucella infection, but the vaccine used is largely uncharacterised. Unfortunately, there is no human vaccine against Brucella and this is a problem given that Brucella are potential bioterrorism agents. Thus, understanding more about Brucella infections could help in the development of a human vaccine and improve the exisiting cattle vaccine. Recent evidence suggest that there are commonalities in the strategies used by the soil bacterium and Brucella to survive long-term within plant and animal/human cells, respectively. My research suggests that structures on the outside of the bacterial cell known as lipopolysaccharides (LPS) are important for long-term survival. By using the soil bacterium-plant interaction as a model system, I identified that the bacterial LPS changes during the interaction with the plant. Thus, the overall goal of this proposal is to characterise these plant-induced LPS changes, to determine how the bacterial cell brings about these LPS changes and to determine whether these LPS changes are involved in the long-term surival of the soil bacterium within plant. The outcome of these studies could not only improve our knowledge of the symbiosis but could also provide insights into how bacteria can surive long-term within animal and human cells.