bacteria

Since experts disagree about whether viruses are “alive,” or, much more controversial, whether prions are alive, the smallest indisputably living things are bacteria. Many of the spherical bacteria are about 1 micrometer in diameter, which is about a hundred times the size of the virus that causes poliomyelitis, and about two and a half times the size of the virus that causes parrot fever.

Smallest bacteria

The smallest known bacteria belong to the type called phytoplasma, which infect plants much as the mycoplasmas infect animals.

Many bacteria are shaped like a gelatin medicine capsule. One such organism, Haemophilus influenzae (it was once thought to cause influenza) is among the smallest bacteria: 0.2–0.3 by 0.5–2.0 micrometers, which is about the smallest object that can be seen through a light microscope. Escherichia coli, the bacteria used in testing water for fecal contamination, has a similar shape and is about 7 μm long and 1.8 μm in diameter.

Many bacteria are long and slender, only 1 or 2 μm wide but 10 to 20 μm long. For comparison, a human red blood cell is 8 μm in diameter.

Largest bacteria

Many bacteria are long but narrow. Some of the larger species are Schaudinnum bütschlii, 50 to 60 micrometers long and 4 to 5 μm thick and Spirochaeta plicatilis, 250 μm long by 0.75 μm thick. A typical very large rounded bacterium, Achromatium oxaliferium is about 100 μm by 45 μm.

In 1993, DNA testing revealed that Epulopiscium fishelsoni, an organism that only lives in the guts of a brown surgeonfish found in the Red Sea, is an extraordinary kind of bacteria.¹ Individual specimens have been measured at 80 mm thick and more than 600 μm long (more than half a millimeter, making them visible to the naked eye). In cell volume, E. fishelsoni far exceeds any other known bacteria. Similar species are found in the guts of surgeonfish on Australia's Great Barrier Reef. It had previously been assumed that prokaryotes were bound to be small because the materials could only travel through the cell by diffusion. (Eukaryotes have ways of actively transporting materials within the cell.) On this assumption, microfossils had been classified as eukaryotes solely because of their size.

1. Esther R. Angert, Kendall D. Clements and Norman R. Pace.
“The largest bacterium.”
Nature, volume 362, pages 239-241 (18 March 18 1993).

See also:
Mitchell L. Sogin.
“Giants among the prokaryotes.”
Nature, volume 362, page 207 (18 March 1993).