Two C57BL/6 mice colonies maintained in two rooms in the same specific pathogen free (SPF) facility were found to have different gut microbiota and a mucus phenotype specific for each colony. The thickness and growth of the colon mucus was similar in the two colonies, but one colony had mucus not penetrable to bacteria or bacterial-sized beads, similar to what occurs in free-living wild mice. On the other hand, the other colony had an inner mucus layer that was penetrable to bacteria and beads. These different properties of the mucus in the two rooms were dependent on the microbiota, as the phenotypes were transmissible by transfer of ceacal microbiota to germ-free mice. Mice with an impenetrable mucus layer had increased amounts of Erysipelotrichi, while mice with a penetrable mucus layer had higher levels of Proteobacteria and TM7 bacteria in the distal colon mucus. Thus bacteria affect mucus barrier properties in ways that can have implications for health and disease.