Measuring the Viscosity of Intestinal Fluids of Larval Zebrafish In Vivo Using Switchably Motile Bacteria as Microrheological Tracer Particles

The health of the human gut microbiota is critical to the health of the host, as symbiotic bacteria perform or aid in numerous vital functions like training the immune system and nutrient metabolism. Thus, we are interested in understanding the factors that influence how the microbiota live and thrive in the gut, such as the viscosity of the intestinal medium. Zebrafish have similar physiology to humans, and are optically transparent as larvae, making them effective model organisms. In this project, we attempt to perform passive multiple particle tracking microrheology of immotile bacteria to measure the viscosity of the gut fluid of larval zebrafish. We employ commensal bacteria which robustly colonize the zebrafish gut and possess an engineered genetic switch with which we can induce a loss of motility post-colonization. To our knowledge, the use of bacteria as tracer particles has never been done before, introducing new obstacles but also posing unique advantages for applications in systems to which the addition of synthetic tracer particles is difficult or disruptive. We found through simulations, in vitro tests, and preliminary in vivo trials that our experimental methods are viable, though systemic colonization problems have so far hindered more definitive measurements.