Friday, 4 November 2016

Counting microscopic sheep? Human and microbial cell mix revealed

Pick up a text book on microbiology or human physiology. Chances are you’ll stumble across a reference to the number of microorganisms in the human body exceeding the number of cells ten-fold. But is this correct?

The microorganisms together with the associated genetic components found within the human body, constitutes what is known as the “microbiome” (an alternative name is “microbiota”, although this tends to be reserved for microorganisms only without the genetic material.) Beginning in 2008, The Human Microbiome Project (HMP) was “set the goal of identifying and characterizing the microorganisms which are found in association with both healthy and diseased humans.”

Within the wider microbiome, there are defined areas, such as the microbiome of the skin, and in the different areas there are ecological niches. So, the types of microbes found on the back (bacteria like Staphylococcus species, for example) is different to the types of microorganisms found on the heel (where there are plenty of fungi.)

Characterizing the microbiome is important. Understanding what is residential or transient to the skin, for example, helps inform about infection control. Knowing what is inside the gut (which is, incidentally, shaped in infancy) can explain patterns of obesity. As Wilder and colleagues put it “microorganisms are believed to influence human physiology through processes related to digestion, immunity, development, and resistance to pathogens.”

The gut microbiome also heralds a new era of personalized medicine (or what the U.S. National Institute of Health calls “precision medicine.”) Here specific treatments can be tailored towards individual patients. This is because different bacteria can affect how medicines are processed by the body.

While the microbiome is important, how big is it? Back in 1972 a biologist called Luckey probably over-stated the number of microorganisms; and in 1977 a researcher called Savage proposed the numbers of microbial cells within the human body out-numbered human body cells by 10 to 1 (although if you extracted the microbiome and freeze-dried it, the resultant pile of dust would only weigh 200 grams.)

New research, from Israel and Canada, turns this oft repeated ratio on its head. The new finding puts forward a more even ratio of one-to-one. To illustrate this, they take a ‘typical man’ as a reference point. Imagine a man who weighs 70 kilograms, is aged between 20–30 years old and stands 1.7 metres tall. This man, Ron Milo, Shai Fuchs and Ron Sender state is made up of around 30 trillion human cells and 39 trillion bacteria. A much closer difference.

It also stands that this ratio periodically slips more in favour of human cells. With each act of defecation, more bacteria are lost from the body than are added to it. Although with the speed bacteria divide at, the numbers drift back some hours later.

The new calculation is based on various cell counts and calculations based on organ volume (using techniques like magnetic-resonance imaging.) This has produced a new ratio of 1.3 microorganisms to every 1 human cell.

The new interpretation of the data is published in bioRxiv. The research paper is titled “Revised estimates for the number of human and bacteria cells in the body.”

Posted by Dr. Tim Sandle

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