The mucosal membrane continues to be one of the most intriguing and vexing components of the gut microbiome. It is the interface between the body and the environment, it is inhabited many bacteria, and it is a nutritional source that shapes the populations in the gut. There is still very little known about the specific interactions between gut mucous and bacteria, but this critical system is rapidly being studied. In the most recent advance, scientists from Switzerland and Germany examined two very different gut bacteria that fill different mucosal niches. They published their results in the journal Nature Communications. The two bacteria they studied were Bacteroides thetaiotaomicron (B. theta) and Escherichia coli. B. theta is a slow growing bacteria that has high metabolic flexibility that is capable of directly using gut mucins as an energy source. E. coli is a fast growing bacteria that is much more limited in its metabolism and can’t directly use the carbohydrates in the gut, but can take hold and rapidly proliferate after a course of antibiotics.
The researchers meticulously researched gnotobiotic mice and made many discoveries about bacteria in their mucous. First, they discovered that the mucosal microbiome varies across its thickness, and is sterile closest to the intestines, but rich in life closest to the lumen. In addition, they noted that the luminal microbiome is distinct from the mucosal microbiome, even though the mucous is constantly being shed into the lumen. To this end, they confirmed that with regards to E. coli, these bugs replicate faster than they are shed (in about 3 hours in the mucous but 8 hours in the lumen), and that their persistence is due to replication rather than uptake from the lumen. How though, can E. coli thrive with their limited ability to break down mucins? The scientists learned that they likely metabolize iron, in addition to atypical carbon sources such as fatty acids and glycerol. B. theta, on the other hand, has a huge repertoire of genes to break down mucins. They do, though, have the ability to leave the mucins and form biofilms on bits of food, such as fiber, that pass through the lumen, and this is one way they travel through the gut. Regardless of whether they are in the lumen or the mucins they proliferate at the same rate.
Each of these bacteria occupy different niches in the gut, and each is important to our health. The discovery that E. coli can use iron for metabolism is particularly interesting, as chemotrophy is not normally considered as important in the body, and may be important to iron regulation. As more research is published the mucous appears to be ‘where the rubber meets the road’ in the microbiome, and new discoveries in this area will be crucial to our overall understanding of the microbiome’s interaction with the body.