The name "microbe" suggests creatures that are so tiny you need a microscope to see them. For most microbes this is true, but there are some gargantuan microbes that we can actually see with the unaided eye. On this page, you’ll read about the biggest of the big in each of the main categories of microbes: viruses, bacteria, protists, fungi and, as an extra added bonus, parasitic worms.
Viruses are super tiny. There’s not a single type of virus that can be seen without a microscope and regular light microscopes that allow us to see bacteria and protists aren’t powerful enough to view viruses. Scientists use special types of electron microscopes to see viruses.
However, viruses come in a wide range of sizes and there are some biggies among them (big being a very relative term here). The largest of them all are a group called the Filoviridae fill-oh-vir-ih-dee. This family includes the Ebola virus, which you might have heard of. Filoviridae are long, thin viruses that look like bent tubes. They can reach up to 1, 000 nanometers or 1/10, 000th of a centimeter in length. This makes them actually longer than some of the smaller bacteria. However, these long viruses are only about 80 nanometers in diameter.
Filoviridae are big for viruses, but they are mere pipsqueaks next to the largest of the bacteria.
The colossus among bacteria is a single-celled giant that lives in the ocean and is named Thiomargarita namibiensis thigh-oh-mar-ger-ee-tuh nah-mih-bee-en-sis, which means "sulfur pearl of Namibia." It was found in the ocean floor off the coast of Namibia in Africa. T. namibiensis’s ball-shaped cells can grow to almost 1 millimeter or 1/25th of an inch in diameter. That’s about as big as the period at the end of this sentence. Size-wise, the largest T. namibiensis cell is to an ordinary bacterium what a 75-foot (23-meter) blue whale is to a newborn mouse. Wow!
T. namibiensis "eats" sulfur and "breathes" nitrate. It stores these molecules in bubble-like compartments in its cell called vacuoles vac-you-ohls. These vacuoles take up 97 percent of the space inside the cell and give the bacterium a pearly, blue-green color. Basically, these vacuoles act like food storage and scuba gear, enabling the bacterium to survive long periods when sulfur and nitrate are scarce without "starving" or "suffocating."
A bacterium that’s big enough to be seen without the aid of a microscope is pretty amazing. But it’s a mere dust speck when it comes to the largest protozoan…
Giants Among Protozoa
The biggest single-celled protozoa are ocean dwelling creatures called foraminifera for-am-an-if-air-uh or forams for short. While many species of foraminifera don’t grow over a single millimeter in diameter, the largest forams can reach sizes of 5 to 6 centimeters (a little over 2 inches) in diameter. (Fossils of ancient forams have reached up to 15 centimeters!) Holy microbe! How can a single cell get that HUGE? It’s thanks in part to shells.