DISSERTATION RESEARCH: Quantifying Nitrogen Limitation in the Gut Microbiota

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The availability of different types of nutrients can determine which species are able live together. When microbes (such as bacteria) live inside animals (such as humans), they may compete for a limited nutrient (such as nitrogen) or may cooperate with each other to acquire that same nutrient. This project will examine how humans and the bacteria in their guts manage their needs for nitrogen. Humans get nitrogen from their diet, and gut bacteria get nitrogen from inside their human hosts. The bacteria can either compete with humans for nitrogen in swallowed food or they can be "fed" through material that humans excrete into the gut. The researchers will focus on competition for nitrogen in the gut and test whether it is important in determining which species of bacteria can live there. The results of this work may help in the design of treatments that require a change in the community of bacteria living in the guts of sick people and domestic animals.The hypothesis that nitrogen is limiting in the gut comes from the observation that most indigestible material reaching the large intestine is nitrogen poor and, therefore, there is little nitrogen available to most of the gut's microbiota. This leads to two testable predictions. First, there is a longitudinal gradient in the environmental (non-microbial) carbon-to-nitrogen (C:N) ratio and a depletion of diet-sourced nitrogen along a healthy gut. Second, much of the nitrogen used by bacteria must come from host secretions. These predictions will be tested by measuring C:N ratios along the gut and by using stable isotope tracer studies to identify the fate of dietary nitrogen and the source of bacterial nitrogen. Finally, to determine if changes in nitrogen availability can alter microbial community composition in the gut, in vitro co-culture competition experiments will be conducted under varying levels of nitrogen availability.