Microbiome … a word in transition

As knowledge progresses, new words enter specialized areas of language (e.g., science, healthcare, IT, etc.) and may gradually — over time — become assimilated into the common vocabulary of the “man or woman in the street”.

Past examples of such changes in language include words like “vaccine” and “transplant” in medicine; “asteroid” and “satellite” in science and astronomy; “program” and “app” in IT; etc.

A word that seems to be going through that transition at present is the term “microbiome”, defined by Merriam Webster as either

  • a community of microorganisms (such as bacteria, fungi, and viruses) that inhabit a particular environment and especially the collection of microorganisms living in or on the human body or
  • the collective genomes of microorganisms inhabiting a particular environment and especially the human body

It is the first of these two definitions that is currently most likely to be  familiar to an expanding range of people. It can be first identified as being used to have that meaning in 1952. The second is a more recent use and is still a definition more familiar to those who live in the “work world” of the biological sciences.

However, what the average “man in the street” probably doesn’t yet appreciate is the extraordinary relationship between a complex animal (be that an elephant or a human) and that individual animal’s microbiome.

First, just as every complex’s animal’s genetic makeup (its DNA) is unique, so is its microbiome. Identical twins don’t have the same fingerprints and they don’t have the same microbiomes either. In fact we now know that they don’t even have the same DNA (although the differences in the DNA of two identical twins will be very small by comparison with the differences in the DNA of two brothers or two sisters born to the same parents).

Why is this?

Because each of us builds our microbiomes over time, starting with the absorption of bacteria, viruses, and fungi carried by our mother in her body while we are still in our mother’s uterus, and continuing to do so after birth, as we go through all the very normal activities of life: eating, breathing, sleeping, kissing, having sex, rolling in the mud, swimming and sunbathing in the summer, skiing and snowboarding in the winter, you name it. And those two identical twins will have differing microbiomes by the time they are born — quite simply because they have differing positions in their mother’s uterus and therefore they will be exposed to slightly different levels of microorganisms over their 9 months in utero.

The other thing most average “men and women in the street” don’t appreciate is the sheer number of  microorganisms in the average human microbiome. The most recent research seems to suggest that there are about 37 billion individual human cells in the average healthy adult, and they are matched by about the same number of microorganisms — or maybe somewhat more. (The older idea that we had about 10 times as many microorganisms as human cells was based on a  massive underestimate of the number of cells in the average human body. More recent research suggests that the ratio of microorganisms to human cells is somewhere between 3 : 1 and 1 : 1.)

Individual human health is profoundly impacted by the “health” of our individual microbiomes. We couldn’t function at all without most of these microorganisms. We live with them in complex symbiotic relationships. They need us and we need them. And when an individual human microbiome becomes unbalanced (because of a significant rise or decrease in the numbers of specific types of microorganism normal to the human microbiome) we may become ill or even die.

The careful management of the microbiome of each individual over his or her lifetime may hold the key to long-term human health, but that doesn’t for one second imply that taking “probiotics” is necessarily a good idea at all. Such products may actually disturb the balance of an individual’s microbiome in very negative ways.

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