In a previous blog post, I discussed aspects of the human microbiome – primarily bacterial roles in our digestion and a few comments on interactions between bacteria and our immune system development. To build upon that, I would like to go further into details that indicate a coevolution between us and our bacterial friends.
Studies indicate that disruption of the naturally occurring bacterial species during infancy increases the risk of auto-immune disorders. This led to the Hygiene Hypothesis. This hypothesis claims that, because we evolved with these bacteria in and around us, our immune systems have compensated for their presence. Bacterial populations survive by suppressing our immune systems. We, in turn, overproduce immune cells so that we can still fend off serious infection. Specifically, we produce high amounts of invariant killer T-cells (INKTs) which stimulate many other immune cells. When naturally occurring bacterial populations are removed, our immune systems still find something to attack. Unfortunately, that something is our own cells. As we continue to be more and more protective of our offspring and their cleanliness, we actually increase their risk of developing auto-immune disorders. I would still recommend against rolling your child around in the mud, but if they get a little dirty exploring it may actually be beneficial.
Another interesting look into our coevolution with bacteria is human breast milk. Before baby formula and bottle feeding, breast feeding was the best, if not only, option for nourishing a newborn child. About ten to fifteen percent of human breast milk consists of indigestible oligosaccharides. They appear completely intact in the infant’s urine. This seems rather wasteful since the child cannot use them as an energy source. However, healthy bacterial populations thrive on these oligosaccharides. Additionally, beneficial bacteria appear in mammary glands only during lactation. The mother provides part of her microbiome for her infant as well as the energy source for the bacteria to thrive. Although baby formula companies have attempted to reconstruct these naturally occurring oligosaccharides, results have generally been mixed. These results indicate that, if possible, a mother should breast feed her child. We seem to have evolved for this to be beneficial to our survival beyond simply acquiring energy to grow.
Looking into the digestive tract provides a third aspect of our coevolution with bacterial populations. When people have diseases, such as cholera, that cause massive water loss via diarrhea, it is extremely important to ensure they remain hydrated. A mixture of sodium and glucose in water served this purpose well, but there was still significant water loss through the large intestine, a region of the digestive tract where a large amount of water reabsorption normally occurs. Prolonged use of antibiotics also resulted in diarrhea and subsequent water loss. Research revealed that there are bacterial species in the large intestine that produce short chain fatty acids which enhance sodium and fluid reabsorption. Starches that are resistant to digestion in the small intestine help stimulate rehydration by providing a food source for the bacterial species.
Looking at bacteria as partners in evolution is not the first notion that people may have, but it is accurate. We won’t be able to truly understand our health and many of our health problems unless we assess them with this thought in mind.
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