Coalition of Israeli scientists reevaluate estimate of bacteria in human microbiome

Published: February 25, 2016

Staff Writer

Our microbiome consists of all the microorganisms found naturally within our body.

Microorganisms are important and vital in many of our organ systems. Bacteria can be found in the gastrointestinal track, skin, saliva, and oral mucus. In 1972, Thomas D. Luckey, Ph.D., published an article stating that there were 100 trillion microbes in the gut and an additional one trillion on the skin. After Luckey made his claim, D.C. Savage, Ph.D., published an article in 1977. He used Luckey’s estimation and claimed that there was a 10:1 ratio of bacteria to human cells. This 10:1 ratio has frequently been reported, and Savage’s article and this figure have been cited in over 1,000 research papers.

Although Savage’s estimate was catchy and helpful at the time, it was never meant to be taken as an accurate estimation. Additionally, this ratio was calculated with a “reference human” of a man 20 to 30 years old, who weighs about 154 pounds and is five feet six inches tall. The ratio did not control for age, gender and obesity.

An article published in 2013, stated that there were 37.2 trillion human cells. Therefore, using the 10 to 1 ratio, there would be 372 trillion bacteria cells in the body. About a month ago, Israeli scientists published an article challenging the 10:1 ratio and claiming that bacteria only outnumbered human cells 1.3 to 1. This almost 1 to 1 ratio was achieved using many mathematical calculations and models to estimate both the number of bacteria cells and human cells in the body.

To determine the total number of human cells, the researchers examined every cellular component, and 56 unique cell types were identified. Red blood cells made up about 84 percent of the total cells in the body. About 90 percent of the human cells originated as hematopoietic stem cells, or stem cells that give rise to red blood cells, even though most of the body, by mass, consists of muscle and fat. After the number of human cells was estimated, researchers used the volume of the colon and the number of bacteria cells able to live per liter of fluid to estimate the number of bacterial cells.

This updated study also looked at variables such as age, gender and weight as factors that might affect the ratio of bacteria to human cells in the body. The study found that females have a lower number of red blood cells and a lower blood volume, both of which contribute to a smaller number of human cells in the body. This results in about a three times increase in the bacteria to human cell ratio in females. Similarly, as one’s body weight increases, so too does their blood volume, resulting in more human cells. However, this difference is not as dramatic and it is accounted for in the standard deviation of the 1 to 1 ratio. Other factors also affect the relative abundance of bacteria in the body. For example, defecation excretes approximately one-third of the colonic bacteria at a time, resulting in a ratio that may have more human cells, until the colon replenishes itself.

Although this new study dramatically changes the perception of the microbes in the human body, it does not diminish the important role that microbes play in the health of humans. They play important roles in nutrition, metabolism and the immune system. Having an accurate estimation of the number of microbes will only aid in further research to study the microbiome of the human body.

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One Response to Coalition of Israeli scientists reevaluate estimate of bacteria in human microbiome

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