The Human Microbiome

The human body is composed of something like 30 trillion cells. Those are are own, made by ourselves from our DNA. Living on and in us however is another 40 trillion or so cells made up of a slew of bacteria, mainly in the gut. The microbiota include viruses, fungi and protozoa. In aggregate, they occupy nearly all other body cavities such as the mouth or vagina in addition to every square inch of our skin. The average forearm alone is home to over forty different species of bacteria.

Since Louis Pasteur expounded on the germ theory of infectious disease, the common idea has been that the only good bug is a dead bug. Yes there are bacteria that can cause toxic effects and some people go to great lengths to try to sterilize their bodies using alcohol or antibacterial detergents and lotions. Ironically, irregular use of these agents may be breeding resistant organisms and hence doing more harm than good.

The vast majority of our micobiota do not cause acute diseases. And in fact recent research suggests that we may well be able to improve our health by fine tuning just which of these trillions of bacterial cells there are. In 2007 the National Institutes of Health began a effort to study all aspects of the human microbiome as it relates to health and well being. The effort involves studying the genes of the microbiota, but in so doing scientists are learning of just what the bacteria are doing. Some very interesting results are turning up.

It has become apparent that the microbiota in the gut of a neonate are very important in training the immune system. Essentially our immune system needs early examples of non-self cells to “learn” how to properly react. There are many autoimmune diseases such as juvenile diabetes and rheumatoid arthritis where our immune system fails to properly distinguish between self and non-self, resulting in inappropriate attacks on our own cells.

Interestingly, there may be connections between the gut and our mental health. There is a correlation between conditions such as irritable bowel syndrome or Crohn’s disease and psychiatric conditions like anxiety and depression. A 2009 publication from the NIH noted, “While evidence is still limited in psychiatric illnesses, there are rapidly coalescing clusters of evidence which point to the possibility that variations in the composition of gut microbes may be associated with changes in the normal functioning of the nervous system.” In mouse models, they found both behavioral and brain chemistry differences between normal mice and those raised in a environment free of bacteria.

The real promise in this research follows from the fact that our microbiome is or can be quite variable between birth and death. Both the variety and number bacteria can change over time periods from days to years. A recent application involves “fecal transplants.” Some patients who have been treated with strong antibiotics have had their colons overgrown with C. difficile, a harmful bacteria. Transfers of normal microbiota from a healthy donor allows for the repopulation of the recipient’s colon and the elimination of the harmful bacteria.

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