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Well, the smallest of the small are the viruses. They’re so tiny, we need a special kind of microscope called an electron microscope to see even the largest of them.
Viruses come in a range of tiny sizes, but the smallest of all are members of a group called the Parvoviruses <par-voh-vye-rus-iz>. Some of these spherical viruses can be as little as 18 nanometers in diameter.
How small is that, you ask? Well, a nanometer (abbreviated nm) is one-billionth (1/1,000,000,000th) of a meter. There are about 25,000,000 nanometers in 1 inch.
To give you a comparison, an itty-bitty E. coli bacterium is a giant next to a parvovirus. This capsule-shaped bacterium is about 2,000 nm long and 500 nm wide. A single hair from your head is anywhere from about 17,000 to 180,000 nm in diameter, depending on what kind of hair you have. Even though a hair sounds gargantuan compared to a parvovirus, just try staring down at the tip of one of your hairs—you’ll go cross-eyed trying to focus on its tiny width. (To get an even better sense of how different microbes compare in size to a hair, try the Let's Get Small activity in the Experiments section.)
So parvoviruses are pretty darn tiny! But are they the tiniest living things? Well, since it’s uncertain whether you can call viruses "living" or not—and at the very least viruses definitely aren’t free-living things since they can’t reproduce on their own—we have to give the crown for smallest known free-living things to a group of bacteria called the Mycoplasmas <my-coh-plaz-muhs>. Some of the little bacteria in this family measure only about 150 nm wide. They are actually smaller than some viruses. There are some 90 different Mycoplasmas out there, some of which infect animals, others plants and others insects. Fourteen are known that infect people.
Now, notice I said that Mycoplasmas are the smallest "known" free-living things. There may be even tinier bacteria out there. In 1998, a team of Australian scientists announced that they had found incredibly tiny bacteria in ancient sandstone dredged up from an oil drilling site about three miles below the ocean floor off of western Australian (see this news article).
Viewed through powerful electron microscopes, the things they found look like fuzzy tangles of threads. Individual "threads" measure only 20 to 150 nm in length. At the largest, that would make them the size of the smallest Mycoplasmas. At the tiniest, that makes them no bigger than little viruses. The researchers nicknamed their find "nanobes."
Because these nanobes are so incredibly tiny, they are devilishly hard for scientists to work with. But the Australian scientists reported that they have seen these evidence that these little things reproduce and grow and their studies have shown signs of DNA, the genetic recipe of all living things, in them. Also, when the scientists cut through some of the tangles, they saw different patterns that looked like the outside and inside of a cell.
Still, other scientists are doubtful that these nanobes are living things. After all, as some scientists point out, proteins and molecules—all the tools and stuff needed for a living cell to function—take up space. A single ribosome <rye-boh-zome>, a molecule that reads the DNA recipe and builds proteins from it, would fill up a sphere 50 to 60 nm in diameter. So how could you have a free-living cell that uses DNA as its code of life that is only 20 or 30 nm wide?
The scientific jury is still out on whether these nanobes are truly free-living super-tiny bacteria or not. That means that officially, Mycoplasmas still get to claim the title of smallest living things on Earth—for now.