Our Microbiome: How Human Are We?
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Referred to by the National Human Genome Research Institute as one of the greatest feats of exploration in history1, the Human Genome Project (HGP) was launched as an international, collaborative research program focused on mapping and understanding the totality of our genetic make-up. The full human sequence was published in 2003, but rather than an endpoint, those outcomes marked the beginning of many new fields of endeavor. One of the natural extensions of the HGP focused on the human microbiome, which remains a dynamic field of investigation to this day.
Until quite recently, the most commonly cited estimates describing the ratio of bacteria to human cells led us to believe the microbes outnumbered us 10:1. But we may not be quite as overwhelmed as once thought. In a 2016 study, Revised Estimates for the Number of Human and Bacteria Cells in the Body,2 researchers revised that estimate downward.
The original calculations of approximately 1014 resident bacteria, dating back to the 1970s, are based on an assumed alimentary tract volume of 1 liter with a consistent 1011 bacterial density per mL. Sender and colleagues believe that both the volume and bacterial density factors were overestimated. Based on stool volume studies, as well as MRI image analysis, they concluded that the alimentary tract volume of a “reference adult male” is closer to 0.4L. Moreover, our advanced understanding of the microbiome informs us that bacterial density varies by location in the alimentary tract and cannot be estimated using a single density factor. Taking this new information into consideration, the authors calculated that the average male hosts a population of 3.8x1013 microbial cells.
The next step was to revisit estimates for total human cell count. Taking into account various cell types and populations, the investigators came to the conclusion that the total number of human cells in their “reference adult male” equates to 3.0x1013 with a calculated 2% uncertainty and 14% coefficient of variation (CV).
In short, the latest calculations describe the ratio of bacteria-to-human cells as about 1.3:1, making us just a little less than half-human on a cellular basis. Although the sheer numbers remain breathtaking and emphasize the significance of our microbial guests, it is somehow reassuring to know we aren’t quite as outnumbered as once thought.
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