The mucus lining of our intestines are the critical interface between the microbiome and the epithelial cells that make up the intestines. It provides many essential roles in its interaction with the microbiome, but perhaps most importantly it is a physical barrier that separates microbiome bacteria from the vasculature. Without it, bacteria can elicit an immune response which results in inflammation that is characteristic of IBDs. New research out of Sweden shows how mucus changes over time as the result of microbiome colonization. The results were published in the journal Cell Host and Microbe last week.

The researchers undertook a number of experiments whereby they measured the mucus in germ free and conventionally raised mice. In general, the conventionally raised mice had thinner, more easily shed mucus in their small intestines that allow for diffusion of nutrients. This mucus contained antimicrobial peptides that prevented bacteria from passing across it. The conventional mice’s large intestines’ mucus was thick and stiff and impenetrable to bacteria. This mucus maintained most of its properties even after the conventionally raised mic were treated with antibiotics. On the other hand, the germ free mice had thin and stiff mucus in their small intestines, as well as thick, but easily penetrable mucus in their large intestines. This shows that the conditions under which the mucus develops is important to its eventual structure and function. After, the scientists inoculated some of the germ free mice with the microbiome of the conventional mice and monitored the mucus over time. It took an entire 6 weeks for the mucus to finally resemble the mucus of a conventionally raised mice. In addition, the scientists looked that the glycans that were being formed in the mucus, and also noticed differences between the germ free and conventional mice. As discussed on this blog previously, these glycans can be important in determining which bacteria colonize the gut. To that end, the researchers measured the microbiome in the mice and discovered that conventional mice had a higher Firmicutes to Bacteroidetes ratio compared to germ free mice. In addition, even after the germ free mice had been inoculated with the new bacteria, their microbiomes never truly matched the conventionally raised mice’s.

More than anything, this paper shows us the critical importance that mucus plays in microbiome health, science and research. It demonstrates the importance of an early life microbiome to the maturation of a healthy mucus that can properly regulate the microbiome. It also shows a possible negative consequence of antibiotics or dietary compounds can have on the mucus, and by extension the microbiome. Finally, among many other things, it shows that microbiome research should consider the effect of the mucus on their experiments, especially ones involving germ free mice.