Mutualism of the Month: You're eating for 100 trillion, cross-feeding microbes and their humans

The major components of the human digestive system. Your mouth, esophagus, stomach, and small and large intestine are colonized by bacteria. Photo courtesy of wikimedia.org.

The major components of the human digestive system. Your mouth, esophagus, stomach, and small and large intestine are colonized by bacteria. Photo courtesy of wikimedia.org.

You may have heard there’s more bacterial cells inside you than human cells; it’s true. There’s about 10 times as many bacterial cells (~100 trillion) compared to human cells (~10 trillion). It’s estimated that there may be up to 1,000 different species of bacteria living in the human gut! Gut microbiota (not flora, flora are plants) make up to 3% of your total body weight. Along with the multitude of bacteria, your gut plays host to fungi, protozoans, and archaea. The highest diversity is found within the large intestine. Outside of the large intestine, various attributes of other parts of the digestive system are harsh and, as a consequence, contain less diversity. The stomach and upper small intestine are highly acidic and bile, released in the small intestine, contains antimicrobial chemicals. As there is little oxygen in the lower digestive system, anaerobic bacteria are the predominant group, not needing oxygen to carry out their necessary functions.

Escherichia coli is abundant in your lower gut. Most strains of E. coli are benign, but several strains are highly pathogenic. Photo courtesy of wikimedia.org.

Escherichia coli is abundant in your lower gut. Most strains of E. coli are benign, but several strains are highly pathogenic. Photo courtesy of wikimedia.org.

Your microbiota are completely dependent on you (the host) for food. If you don’t eat, food becomes scarce for them too. One service they provide for you is digesting normally indigestible carbohydrates (dietary fiber) such as cellulose and pectin into fatty acids through fermentation (the conversion of sugars into acids, gases, and/or alcohols). Some of the fatty acids produced are then released back into your intestine where they are absorbed. Particularly helpful fatty acids, produced by bacteria, include acetate, propionate, and butyrate, which are utilized by your colon (butyrate), the liver (propionate), and muscles (acetate).

Along with digesting unusable fiber into usable fatty acids, bacteria are known to synthesize vitamins B1, B7, B9, B12, and K. B vitamins aid in energy production while vitamin K is essential in coagulation of blood.

Enterobacter cloacae, another common gut bacteria growing in a Petri dish. Photo courtesy of wikimedia.org.

Enterobacter cloacae, another common gut bacteria growing in a Petri dish. Photo courtesy of wikimedia.org.

Enzymes produced by intestinal bacteria play a critical role in enterohepatic circulation. As simply put as possible, enterohepatic circulation involves the liver excreting modified chemicals that need to be reabsorbed by the intestines after being used in digestion. These substances are then recycled by the liver and used in digestion again. In their liver-modified state, these substances are poorly reabsorbed by the intestine. Enzymes, produced by the intestinal bacteria, change the shape of the substances, allowing for easy absorption by the intestines.

This exchange of substances between you and your microbiota has such a large impact on how your body functions that irregularities can actually create common ailments. Certain antibiotics can block the enterohepatic circulation mentioned above by suppressing the bacteria. This reduces the absorption of the modified substances, which end up being passed out of your system unutilized. For example, blood levels of estrogen from birth-control pills decrease when antibiotics are administered and this is directly tied to the enterohepatic circulation. Some people lack intestinal lactase (“lactose intolerant”), the enzyme responsible for breaking down the sugar lactose in milk. People deficient in lactase don’t adequately digest and absorb lactose before it gets to the large intestine, which by itself isn't a problem. However, the lactose that reaches the large intestine goes through vigorous bacterial fermentation producing large amounts of gas leading to uncomfortable distention, flatus, and diarrhea.

The lining of your lower intestine is coated in bacteria. Photo courtesy of gastrolab.net.

The lining of your lower intestine is coated in bacteria. Photo courtesy of gastrolab.net.

The relationship in your intestines is fairly stable. The number and types of bacteria in each area of your intestine stays relatively similar across time. The stability of the normal biota discourages infection by potentially pathogenic (disease-producing) members of your gut. Bacteria are actively producing antimicrobial compounds inside you as they constantly compete for space and this competition creates a highly stable environment.

The relationship you have with your gut bacteria is in general mutualistic. You provide them with a stable environment and three meals a day, and in return they provide you essential vitamins and metabolites which keep you running. So, next time you’re eating your lunch and you want one more chip, just think: you aren’t eating for one, you’re eating for 100 trillion!

For some good (science-heavy) reads on the relationship you share with your bacteria check out these links:

http://www.ncbi.nlm.nih.gov/books/NBK7670/

http://www.sciencedirect.com/science/article/pii/S1075996405000685

http://www.annualreviews.org/doi/abs/10.1146/annurev.mi.31.100177.000543

Don’t forget to check out more mutualisms of the month and all the other great articles on ftdm! Stay hungry!

More Mutualisms of the Month