Mucus is a complex biological fluid that plays a variety of functional roles in many physiological systems. Intestinal mucus in particular serves as a physical barrier to pathogens, a medium for the diffusion of nutrients and metabolites, and an environmental home for colonizing microbes. Its rheological properties have therefore been the subject of many investigations, thus far limited, however, to in vitro studies due to the difficulty of measurement in the natural context of the gut. This limitation especially hinders our understanding of how the gut microbiota interact with the intestinal environment, since examination of this calls not only for in vivo measurement techniques, but for techniques that can be applied to model organisms in which the microbial state of the gut can be controlled. We address this challenge by developing a method that combines magnetic microrheology, light sheet fluorescence microscopy, and microgavage of particles, applying this to the larval zebrafish, a model vertebrate. We present measurements of the viscosity of mucus within the intestinal bulb of both germ-free (devoid of intestinal microbes) and conventionally reared larval zebrafish. At the length scale probed (≈ 10 μ m), we find that mucus behaves as a Newtonian fluid, with no discernable elastic component. Surprisingly, despite known differences in the the number of secretory cells in germ-free zebrafish and their conventional counterparts, the fluid viscosity for these two groups was very similar. Our measurements provide the first in vivo measurements of intestinal mucus rheology at micron length scales in living animals, quantifying of an important biomaterial environment and highlighting the utility of active magnetic microrheology for biophysical studies.
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