Are There Really More Microbes Than Human Cells in the Body?

The notion that our bodies host ten times more bacteria than human cells has been a popular belief for years. But is it true?

the origins of the microbial ratio

The concept of the human body hosting more microbes than cells has its roots in early 20th-century scientific discoveries. In 1908, Élie Metchnikoff introduced the idea of intestinal microflora, an insight that gained widespread attention thanks to later publications.

In 1977, microbiologist Dwayne Savage proposed a now-famous ratio: one human cell for every ten bacteria. This estimate quickly became popular in scientific circles due to its simplicity and appeal as a memorable figure. However, modern research suggests this ratio may not be entirely accurate.

rethinking the numbers

Estimating the number of microbes versus human cells is challenging. In 1972, biochemist Thomas Luckey estimated that each gram of human feces contained 100 billion bacteria, leading to an assumption that the digestive tract housed approximately one kilogram of these microorganisms.

This estimation led to the famous ten-to-one ratio when compared to an estimated ten trillion human cells. Yet, this figure was based on broad assumptions and lacked precise empirical evidence.

a critical reassessment

A pivotal moment came in 2014 when microbiologist Judah Rosner questioned the longstanding microbial proportion in a scientific letter. He pointed out that once an idea becomes entrenched in scientific literature, it can be difficult to challenge or revise.

Rosner highlighted inconsistencies in estimating human cell counts and suggested that even if the microbial-human cell ratio was proven incorrect, it would not undermine the significance of microbes for human health.

new findings emerge

1. In 2016, research from Israel challenged the persistent microbe-to-cell ratio.
2. The team led by Professor Ron Milo focused on determining bacterial counts most representative of overall body composition.
3. Their study found discrepancies in previous estimates, particularly regarding fecal bacteria count and overall cell numbers.

Milo’s team applied refined methods, considering factors such as organ-specific bacterial populations and updated fecal weight assumptions (400 grams instead of one kilogram). Their findings indicated around 30 trillion human cells with a revised bacterial-human cell ratio closer to 1.3:1.

a diverse microscopic ecosystem

Bacteria are not the only occupants; our bodies also host viruses and protozoa—single-celled organisms contributing to our complex microbiome ecosystem. While bacteria have been extensively studied as part of intestinal microbiota analysis today, they represent just one component within this intricate system.

• Bacteria remain numerically superior but are complemented by other microorganisms like yeast and viruses.
• The “virobiome” was once believed dominant; however, recent studies estimate viral populations nearly equal those of bacteria within us.

When combined with viruses’ numbers added into account alongside bacterial presence alone outweighing total cellular makeup collectively present among humans despite occasional fluctuations observed during natural biological processes such as defecation impacting balance temporarily favoring cellular predominance instead!