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Guiding Principles of Spatiotemporal Organization in Biological Communities

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The notion of community in biology can take on many forms, from the interactions of intracellular organelles, to multicellular behaviors of normally single-celled organisms, to complex populations of bacterial species within a host. Dissecting the rules governing the function of these communities is a fundamental challenge that requires the integration of computational models, novel experimental assays, and quantitative analysis methodologies. Recent imaging studies of mitochondrial dynamics have implicated a cycle of fusion, fission, and autophagy in the quality control of mitochondrial function by selectively increasing the membrane potential of some mitochondria at the expense of the turnover of others. To study the relative impacts of local interactions between neighboring mitochondria and their reorganization via transport, we have developed a spatiotemporal mathematical model encompassing all of these processes in which we focus on the dynamics of a health parameter meant to mimic the functional state of mitochondria. Our results show that several general principles emerge from the complexity of the quality control cycle. Next, I will discuss how the unicellular, photosynthetic cyanobacterium Synechocystis sp. PCC6803 transduces a light stimulus into directional movement known as phototaxis. We use time-lapse microscopy coupled with quantitative single-cell tracking to investigate the timescale of the cellular response to various light conditions and to characterize the contribution of the photoreceptor TaxD1 (PixJ1) to phototaxis. We show that individual cells respond within minutes to new light conditions, and that movement directionality is conferred only by the current light directionality, rather than by a long-term memory of previous conditions. Finally, I will discuss a pipeline for the assessment of intestinal microbiota localization within mouse gut cross-sections. Using gnotobiotic and humanized (colonized with human microbiota) mice, we demonstrate that elimination of fiber from the diet, which is known to increase microbiota utilization of host mucosal glycans, results in thinner mucus in the distal colon, increased proximity of microbes to epithelium, and heightened expression of the inflammatory marker REG3β. This broadly applicable framework will accelerate the elucidation of the roles of microbiota localization in health and disease.

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