What is the total number of bacterial cells in the “reference man” as found in this study?
3.8.10 13
What is the total number of human cells in a 170 Kg “reference man” found from this study?
There is no mention to a 170kg “reference man” in this study.
Why did the authors undertake this study?
“ The aim of this study is to critically revisit former estimates for the number of human and bacterial cells in the human body. We give up-to-date, detailed estimates where the calculation logic and sources are fully documented, and uncertainty ranges are derived. By updating the cell counts in the body, we also revisit the 10:1 value that has been so thoroughly repeated as to achieve the status of an established common knowledge fact. This ratio was criticized recently in a letter to the journal Microbe, but an alternative detailed estimate that will give concrete values and estimate the uncertainty range is needed. Here, we provide an account of the methodologies employed hitherto for cell count and revise past estimates. Doing so, we repeat and reflect on the assumptions in previous back-of-the-envelope calculations, also known as Fermi problems. We find such estimates as effective sanity checks and a way to improve our quantitative understanding in biology.”
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What is the “Reference Man”?
"Reference Man is defined as being between 20–30 years of age, weighing 70 kg, is 170 cm in height.”
Why is there no Reference Woman?
“ A major part of the available literature used in the derivation of human cell numbers was based on cohorts of exclusively or mostly men, and as we use these sources, our analysis starts with adult men. As discussed below, relatively modest quantitative differences apply for women due to changes in characteristic body mass, blood volume, and the genital microbiota.”
How did the authors revisit the question of the number of bacteria?
“ We begin our analysis by revisiting the number of bacteria through surveying earlier sources, comparing counts in different body organs and finally focusing on the content of the colon. We then estimate the total number of human cells in the body, comparing calculations using a "representative" cell size to aggregation by cell type. We then contrast the cell number distribution by tissue type to the mass distribution. In closing, we revisit the ratio of bacterial to human cells and evaluate the effect of gender, age, and obesity.”
What were the assumptions underlying the original “back of the envelope” calculation on the number of bacteria in the body?
That there about 1014–1015 bacteria in a normal human body.
8. A report from the NIH stated a value of 1-3% of body mass is composed of bacteria (MacDougall, 2012). This value quoted by Wikipedia (Wikipedia 2015) among many online resources, coupled with a rule of thumb for bacterial cell volume of 1m3 leads to an estimate of how many cells in the human body 1014–1015, and a ratio of bacterial to human cells of 10:1?
What information is shown in table 1?
“ Typical order of magnitude estimates for the number of bacteria that reside in different organs in the human body.”
The concentration of bacteria in the stomach and the upper 2/3 of the small intestine is (lower or higher) than found on the skin? Why?
The concentration of bacteria in the stomach and the upper 2/3 of the small intestine is lower “owing to the relatively low pH of the stomach and the fast flow of the content through the stomach and the small intestine.”
Based on table 1 which organ contributes the most number of bacterial cells to the total number of bacteria in humans?
Colon
What two assumptions underlay the original back of the envelope estimate for the number of bacteria in the colon?
“ The volume of the alimentary tract to be 1 liter.”
“ The density of bacteria is about 1011 bacteria per gram of wet content.”
What is shown in table 2?
“ Table 2 reports the values we extracted from 14 studies in the literature and translated them to a common basis enabling comparison.”
What human cell makes the largest contribution to the overall number of human cells?
Red blood cells
The number of glial cells /( a type of supporting cell) had been previously reported as 3.10 12 , and was based on an estimate of the ratio of glial cells to neurons in the brain had been previously estimated at 10:1 .
The authors found that non-blood human cells constituted only 10% of the total updated human cell count.
Based on the results of their study, the authors revised the estimates for the number of human cells (H) to body mass and the number of bacterial cells (B) to nucleated human cells for a ratio of bacteria to human (B/H) of 1.3 .
To generalize the results of looking at the number of human and bacteria cells in the “reference man”, the authors generalize their conclusions to other populations. In doing so, they identify four factors that dominate the calculation. These factors are:
Colon volume
Bacterial density in the colon
Blood volume
Hematocrit (i.e., red blood cells per unit volume).
In the discussion the authors identify several “knowledge gaps” that may influence the results of their study these knowledge gaps are:
The lack of certainty in using “measured fecal bacteria density to represent the average bacteria density in the colon.”
Limited information on the volume of the colon content across individuals and conditions
The authors provide an example of how knowing the number of cells in different tissues can be an important indicator in understanding variation in cancer risk among tissues. What is the title of the paper referenced in this example?
Variation in cancer risk among tissues can be explained by the number of stem cell divisions.
