Humans and Their Bacteria
by Tiffany Straza
Broadcast 4.21 & 4.24.2020

Cyanobacteria. NASA photo (public domain image).



I consider myself to be a fairly normal human. Ten fingers, ten toes, two ears. Made up of thousands of different cells. Oh, and of all the cells in and on my body, the human cells are outnumbered tenfold by bacterial cells.

Wait, did we get that right? Ten times more bacterial cells than human cells share the human body? Yes, that is indeed the average. Like many other relationships that have evolved over time between species, humans and bacteria form a symbiosis, or “living together,” that is usually beneficial for both us and for them. This particular symbiosis is very important when we consider bacterial ecology.

Bacteria are single-celled organisms, and unlike our cells, they do not have a nucleus. They carry everything they need to survive in one cell, and they manage quite well. Bacteria are found in every place on the planet we have looked so far, and the human body is one very common bacterial habitat.

Where are all of these bacteria in and on our bodies? The answer is: pretty much everywhere. Bacteria can live on our skin and our teeth as well as inside us, and not bother us at all. Many types of bacteria even help us. You may already know that some foods like yogurt and cheese are actually made using bacteria. We use bacteria to create nutritious food that tastes good to us. Well, we also use bacteria inside our body to help digest the food that we eat. Bacteria help humans digest milk products, for example. Some forms of lactose intolerance are related to a person not having enough lactose-digesting bacteria in their digestive system.

When people think about bacteria, we often just think about disease. It’s true that some bacteria do make humans sick. It often happens when “bad” bacteria take over from the “good” bacteria normally present, out-competing them in numbers. But there are many different kinds of bacteria, and less than 2% of bacterial species are harmful to humans. The other 98% carry out their life cycles without hurting people. We wash away excess bacteria as well as viruses when we bathe or wash our hands, but it’s not important to soak ourselves in antibacterial soap all day, because our background populations of bacteria are part of a normal and healthy life.

Bacteria do a lot of good in the world outside of our bodies, too. Some attach to surfaces, but many are free-living. Bacteria recycle nutrients in soil and water, making it possible for new organisms to grow. And did you know that bacteria contribute to primary production through photosynthesis? Generally, when we picture photosynthesis, we think of green trees and grasses, but in fact, half of the world’s primary production happens in the surface of the ocean, dominated by microbes called phytoplankton. The phytoplankton community is made up of single-celled eukaryotes, like microscopic plants, as well as photosynthetic bacteria called cyanobacteria. These bacteria accomplish a quarter of global primary production, using carbon dioxide and energy from the sun to produce the oxygen that we breathe. Think of it this way: take four deep breaths. The first two breaths come from trees, grasses and other plants on land. A third breath comes from the eukaryotes, those tiny plants in the ocean. The fourth breath was provided just by bacteria.

So the next time you wash your hands with antibacterial soap or drink a glass of milk, remember that there are a lot of good bacteria in the world. Some of them are part of making us human.


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