For several decades, it was commonly believed that bacterial cells constituted ~90% of the cells in the human body. You could casually slip this eyebrow-raising fact into a dinner party conversation or a philosophical debate about human identity and the discussion would pause while everyone chewed over that attention-grabber. If we are 90% bacteria then you could argue that humans are basically a minor evolutionary appendage to a seething microbial mass. It was humbling and downright embarrassing, from a species pride viewpoint.
However, about a year ago, researchers from the Weizmann Institute of Science in Israel re-evaluated the data and assumptions behind this eyebrow-raising factoid and poured a pitcher of cold water on it when they concluded that the number bacterial cells in our bodies was only ~30% greater than the number of human cells. Stripping this fact of most of its dignity, the authors pointed out that about 25-30% of bacteria are lost with an average bowel movement, or as they wryly commented “Indeed, the numbers are similar enough that each defecation event may flip the ratio to favor human cells over bacteria.” If that is true then I was at my most human when I underwent a colonoscopy.
But a half-human/half-bacteria hybrid evokes an image of a cheesy monster from a 1950s Grade B sci-fi movie forbidden planet that orbited around my developing childhood psyche. It was a blow to my pride in my species. I am a human, damn it, and I have my biological dignity. I am not some primitive blobby affair that obtains its food by absorbing dead organic material or some thermophile sucking sulfur from a deep Pacific hydrothermal vent. I go to a supermarket to hunt and gather my food like a man! Yeah! That’s what I’m talkin’ about! No foreign species is going to dominate my body!
The lying left wing scientific media got it wrong again. So I pondered how I might further trivialize my bacterial component and fully regain pride in my species. How could I write a scientifically based executive order using alternative biological facts that could ban all foreign species from my native body? And then I hit on the right-in-front-of-me-all-the-time ruby slippers solution: click my heels together three times for DNA, the very currency of evolution! There’s no species like Homo, there’s no species like Homo, there’s no species like Homo. So I asked myself “Hey Bob, you Wizard of Odds genetics specialist, tell me, how much bacterial DNA does the human body contain compared to human DNA?”
The Assumptions
- The analysis of the Weizmann Institute paper is reasonably accurate.
- I used E. coli K-12 as the model organism. Several hundred types of bacteria reside in the human body and some have more or less DNA than E. coli, but E. coli is the predominant bacterial strain in humans.
- The total number of non-bacterial organisms in human – viruses, archaea, fungi – are several orders of magnitude less common than bacteria and are essentially a rounding error of the human microbial makeup.
- The E. coli genome is fairly compact, containing little in the way of introns or non-coding DNA.
- Each E. coli bacterium contains 4,377 genes and 4,639,221 base pairs, which I rounded off to 4.4×103, amid ~4.6×106 DNA base pairs.
- The reference person is a 170cm tall male who weighs 70 kg (Sorry for the sexist bias here, but this is the model used in the published papers. Proportionally though, the ratios here probably apply to all genders, whichever bathroom they choose to use, except in North Carolina).
- Per the updated estimates, the human body contains ~4×1013 bacteria.
- Our bodies contain ~3×1013 human cells. However, per the Weizmann Institute paper, about 90% of those cells are enucleated blood cells. Thus the vast majority of cells in an adult do not contain nuclear or mitochondrial DNA. Ergo, the total number of human cells that contain DNA is on the order of ~3×1012.
- Each nucleated diploid human cell has about 20,000 genes (2×104) and 6,000,000 (6×109) DNA base pairs (though see Addendum below). The number of haploid sperm and egg cells are small enough to ignore for these calculations.
- The total amount of mitochondrial genes and DNA in humans is minor compared to nuclear DNA and can also be ignored for these calculations.
- Unlike bacterial DNA, the vast majority of human DNA is non-coding, resulting in a far higher ratio of DNA to gene in humans compared to bacteria.
The Calculations^
What is the total number of bacterial genes in the human body?
This is calculated by multiplying the number of genes in each bacterium by the number of bacteria in the human body:
(4.4×103) x (4×1013) ≅ 1.7×1017 bacterial genes in the human body
What is the total amount of bacterial DNA in the human body?
This is calculated by multiplying the number of DNA base pairs in each bacterium by the total number of bacteria in the human body:
(4.6×106) x (4×1013) ≅ 1.8×1020 base pairs of bacterial DNA in the human body
What is the total number of human genes in the human body?
This is calculated by multiplying the number of genes in the human body by the number of nucleated cells:
(2×104) x (3×1012) ≅ 6×1016 genes in the human body
What is the total amount of human DNA in the human body?
This is calculated by multiplying the number of DNA base pairs per cell by the total number of nucleated cells:
(6×109) x (3×1012) ≅ 1.8×1022 DNA base pairs in the human body
So to summarize:
| Organism | Total Number of Genes In Human Body | Total Number of Base Pairs in Human Body |
| Bacteria | ~1.7×1017 | ~1.8×1020 |
| Human | ~6×1016 | ~1.8×1022 |
This analysis demonstrates that there are far more bacterial genes in the human body than human genes. The preponderance of bacterial genes is not significantly altered by a “defecation event” or even a colonoscopy prep. The best I can say is that any genetic superiority humans might have over bacteria comes from our “junk” DNA. Not much solace there.
To throw a little salt in the psychic wound, the human genome contains about 150 non-human genes that have insinuated themselves into our double helices. Even some of our human genes ain’t so human. A bit less than 1% of the total, but enough to strike a symbolic blow to the human ego. Homo bacteriensis, I guess. Bacteria rule.
Addendum (Added 3/19/2017)
Actually, in thinking about this for a few more days, I realized that the number of human genes in each diploid cell is ~40,000 since each cell has ~20,000 maternal genes and ~20,000 paternal genes, so the number of human genes in the body is (4×104) x (3×1012) = 1.2×1017. This is getting closer to the number of bacterial genes in the human body, give or take a few quadrillion genes. Likewise, the amount of human DNA in each diploid cell is actually (1.2×1010) x (3×1012) ≅ 3.6×1022 DNA base pairs in the human body. Bacteria, being haploid organisms, only have a single copy of each gene, except just prior to binary fission when their DNA content is doubled. So the bacteria/human differences are greater if you limit the assumption to the number of human genes, not the number of human alleles.
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Thanks to my good friend Tom Wolfe for pointing out to me the revised estimates of bacterial and human cells.
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^ – I freely admit that these calculations and assumptions may not be error free. I ran them several times and kept coming up with different answers. It has been a long time since I multiplied and added exponents. Please check my calculations .
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