Let's Get Small - Which kind of microbe is the smallest, a virus, bacterium or protozoan? Which is the biggest?

Viruses as a group are the smallest microbes. Specific kinds of viruses range in size from a few dozen nanometers up to about 1,000 nanometers or 1 micrometer. Bacteria and archaea are generally the second smallest microbes as a group. But some types of bacteria—Chlamydia, for example—are about as small as the largest viruses. And there are a couple of monster bacteria—Thiomargarita namibiensis and Epulopiscium fishelsoni—that are actually large enough to see without a microscope. These two bacteria are even larger than many single-celled protozoa, which are generally considered among the largest of microbes.


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