Let’s learn about microbes

There may be a billion species of microorganisms on Earth — but scientists have only discovered a small fraction of them.

Let’s learn about microbes

Any unicellular — one-celled — organism is a microbe. Microbes, short for microorganisms, are the largest group of living things on Earth. There may be a billion species of microbes, but only a small fraction has been discovered so far. There are five major groups of microbes:

Bacteria: These single-celled creatures are very simple. They don’t have a nucleus or organelles. Their genetic material is just a loop of DNA. This makes them prokaryotes. Bacteria come in many different shapes. And they can be found pretty much everywhere on the planet. Some of them cause disease.

Archaea: This group was once thought to be just another type of bacteria. Now they’re recognized as their own group. Like bacteria, archaea (Ar-KEE-uh) are prokaryotes. But the genes and enzymes in archaea look more like those of eukaryotes (Yu-KAIR-ee-oats). Those are organisms with cells that have a nucleus. Archaea are often found in extreme environments, like hot springs and salt lakes. But they can also be found much closer to home — such as all over your skin.

Protists: This grab-bag group of eukaryotes includes algae, marine diatoms, slime molds and protozoa. They might live alone or in interconnected colonies. Some can move with the help of paddle-like flagella. Others are stuck in one place. Some, such as Plasmodium, can cause disease. Plasmodium causes malaria.

Fungi: Some fungi, such as mushrooms, are multicellular, and they don’t count among microbes. But single-celled fungi are considered microbes. They include the yeasts that give us bread.

Viruses: Not everyone includes viruses in the microbes. That’s because viruses are not cells. They can’t make proteins. And they can’t reproduce on their own. They need to infect an organism, where they hijack its cellular machinery to make new viruses. Viruses are responsible for many diseases, from the common cold to influenza to COVID-19.

Only a small number of microorganisms are bad for humans — but you should still wash your hands, get your vaccines and take other safeguards to protect yourself.

Want to know more? We’ve got some stories to get you started:

Sweat-slurping ‘aliens’ live on your skin Archaea are famous for living in extreme environments. Now scientists find they also inhabit skin, where they seem to enjoy sweat. (10/25/2017) Readability: 6.7

Bacteria are all around us — and that’s okay Scientists may have identified less than one percent of all bacteria on Earth. But there’s a reason to keep up the hunt. These microbes could help us understand and protect our planet. (10/4/2018) Readability: 7.8

Life on Earth is mostly green A new survey of life on Earth finds that plants and microbes dominate. But even though humans are in the minority, they still play a major role. (3/28/2019) Readability: 7.3

Explore more

Scientists Say: Archaea

Scientists Say: Organelle

Scientists Say: Yeast

Explainer: Prokaryotes and eukaryotes

Explainer: What is a virus?

Cool Jobs: New tools to solve crimes

Analyze This: These viruses are behemoths

Mystery microbes of the sea

Scientists probe new ways to control malaria

Let’s learn about microbial communities


Word Find

The five-second rule implies that if food dropped on the floor is picked up within five seconds, bacteria won’t have time to transfer. Is that true? You can test the five-second rule with an experiment. Check out the experiment’s design, and learn how to build an incubator for growing bacteria and analyze results. Then learn about what other scientists have found.

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Spin in this Milky Way bar may show cosmic dark matter does exist

A method akin to studying a tree’s rings reveals the timeline of a slowdown in those stars at the heart of our Milky Way galaxy.

Spin in this Milky Way bar may show cosmic dark matter does exist

Dark matter can be a real drag. The pull of that unidentified, invisible matter in our galaxy may be slowing down a rotating bar of stars at the heart of our Milky Way.

Scientists used a technique to recreate a timeline for this slowdown. The approach works somewhat like figuring out a tree’s age by counting rings in the trunk. This technique finds the bar’s speed has slowed by at least 24 percent since it formed, billions of years ago.

That slowdown is “another indirect — but important — piece of evidence that dark matter is a thing,” says astrophysicist Martin Weinberg. He works at the University of Massachusetts Amherst. Scientists have long suspected that dark matter exists, even though they can’t see it. The new slowdown is a sign of dark matter, Weinberg says. After all, he points out, “this [slowdown] can’t happen without it.”

Many spiral galaxies, including our Milky Way, contain a central bar-shaped region. It is densely packed with stars. Surrounding it are the galaxy’s pinwheeling arms. The bar also has some groupies: a crew of stars. They sit alongside the bar but farther out from the galaxy’s center. They’re trapped by the bar’s gravitational influence. Those stars orbit about a gravitationally stable site known as a so-called Lagrange point.

Explainer: Stars and their families

If the bar’s rotation slows, it will grow in length. The bar’s tagalongs will also move outward. As that happens, that group of hangers-on will gather even more stars. According to a computer model of this process, those additional stars should arrange themselves in layers on the outside of the group, says Ralph Schönrich. He’s an astrophysicist at University College London in England. The layers of stars serve as a record of the group’s growth. “It’s actually like a tree that you can cut up in your own galaxy,” he says.

Schönrich and Rimpei Chiba of the University of Oxford in England studied how the composition of stars in the group changed from its outer edge to its deeper layers.

Different parts of the Milky Way have different types of stars. Some parts of the galaxy have stars containing hydrogen and helium and not much else. Other parts of the galaxy have stars with heavier chemical elements. By looking at the composition of the layers in the group of stars, the scientists could tell where the group picked up the layers.

They worked with data from the European Space Agency’s Gaia spacecraft. Stars in the outer layers of the bar tend to have few elements heavier than helium, their work revealed. The inner layer’s stars have more of those heavy elements. That’s evidence, they say, that the group of stars is moving outward due to the bar slowing. The group picked up stars of different compositions as it moved along.    

The bar’s slowdown hints that a gravitational force is acting on it. Namely, dark matter in the galaxy seems to be pulling on that bar of stars. Normal matter alone wouldn’t have enough mass to reduce the bar’s speed. “If there is no dark matter, the bar will not slow down,” Chiba says.

He and Schönrich describe their finding in the August Monthly Notices of the Royal Astronomical Society.

But the results have some critics. “This is not yet convincing to me,” says astrophysicist Isaac Shlosman. He works at the University of Kentucky in Lexington. For instance, he doubts that the tree-ring layering would really occur. It is “hard to believe that this is the case in a realistic system” as opposed to in a simple computer model, he says.

Weinberg, in contrast, says that although the study relies on many assumptions, he suspects it’s correct. “It’s got the right smell,” he says.

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