Are we more bacteria than human? That depends when you last pooped

For decades, the notion that bacteria living on our bodies outnumbered human cells 10 to 1 was popular among microbiologists and the public. Turns out, this estimation is wrong.

In 2016, Ron Sender, Shai Fuchs, and Ron Milo from the Weizmann Institute examined the origins of this estimation and found that the ratio is actually closer to 1:1.

Where did the 10:1 microbe:human cell ratio come from?

In 1972, Thomas Lucky published an estimation of the number of microbes carried by an adult man: 10¹² microbes associated with the epidermis and 10¹⁴ microbes associated in the digestive tract. The total number of bacteria in the digestive tract is based on the estimation that 10¹¹ bacteria reside in a gram of feces and that the digestive tract has a total volume of 1 liter. Multiplying 10¹¹ by 1,000 (assuming 1 g = 1 mL) gives you 10¹⁴ microbes in the total digestive tract.

Lucky’s publication also used the total number of human cells as 10¹³, but did not provide a primary reference for this claim.

Using these numbers, the resulting bacteria-to-human cell ratio would be 10:1.

What went wrong in the inital 10:1 microbe:human cell ratio?

Lucky’s calculations assume that the microbial density along the entire digestive tract is the similar. But, on the contrary, it is not. Bacterial density in the non-colonic parts of the digestive tract is negligible compared to the density in the colon. The inner volume of the colon is only estimated to be about 340–480 mL, meaning that there would approximately 3.9 x 10¹³ bacteria from the digestive tract, not 10¹⁴. Considering the contribution of the total bacteria from other organs is at most 10¹² and thus inconsequential, 3.9 x 10¹³ still holds as the best approximation for the number of bacteria in the “reference man.”

As for the number of human cells, Milo’s team focused on the six cell types that comprise 97% of the human cell count: red blood cells, glial cells, endothelial cells, dermal fibroblasts, platelets, and bone marrow cells. After totaling up these cells types, they arrived at 3 x 10¹³ human cells in the “reference man,” similar to the estimates from a few years prior.

With 3.9 x 10¹³ bacterial cells and 3 x 10¹³ human cells, the revised ratio is now closer to 1.3 to 1.

Effects of age, sex, and body weight on colonic bacterial density

The estimations above were based a “reference man” between 20-30 years old, weighing 70 kg, and 170 cm in height. But are things different depending on age, sex, and body weight?

Because bacteria in the colon dominate the bacterial cell count and red blood cells dominate the human cell count, the team focused on colon volume, bacterial density in colon, red blood cell density, and blood volume.

Female and male colon volume are similar and there no report in literature of sex-specific differences in colonic bacteria density. Red blood cell concentration is about 10% lower in females and blood volume is 20-30% lower meaning that the ratio of bacterial cells to human cells increases by a third in females.

Bacterial cell density in the colon is relatively constant from infancy to adulthood, but the blood volume compared to body weight is 10% higher in infants than adults. In elderly however, blood volume is reduced by 25% while red blood cell density is unchanged. This means that the effect of age on the bacterial cell to human cell ratio is smaller than 2 fold from age 1 and onward and within the range for the “reference man.”

The effect of obesity on this ratio is more nuanced. Colonic bacterial concentration is similar to the “reference man,” but colon volume increases with weight…but only to a certain point. Colon volume plateaus at about 600 ml, 50% larger in size than that of the “reference man.” Blood volume also increases with BMI but this increase mirrors the increase for colonic volume. Thus, the bacterial cell to human cell ratio with increased weight seems to be within the uncertainty range of the “reference man.”

While there turns out to be fewer microbes on our bodies than previously thought, these revised numbers do not take away from the importance of the microbiome. So are we more microbe or human? This depends if you’ve pooped recently. As Ron Sender, Shai Fuchs, and Ron Milo wrote, “each defecation event, which excretes about 1/3 of the colonic bacterial content, may flip the ratio to favor human cells over bacteria.”