Mea­sur­ing Up

by Jamie Henzy

One could say that bacteria get quite a bit more attention than viruses here at STC. In fact, one could say that they hog the limelight, chewing the scenery and mugging for the camera while viruses, like stepchildren of a stage mom, hang backstage as understudies. So let's engage in an inclusive family activity, in which both bacteria and viruses can participate. How do they measure up to one another in size as they frolic together on the playground?

Figure 1. How do viruses and bacteria measure up? Adapted from source

Bacteria generally range from 0.3 to 5 microns (one micrometer, or 10^-6 meters), with E. coli a respectable 2 microns in length, and typically approaching 1 micron in width. Most eukaryotic cells would loom over a bacterium, being at least 10 to 20 times wider. But how about a virus stepsibling? Well, viruses of course are generally smaller. Take HIV, for example – a typical particle is about 0.1 microns in diameter, and somewhat spherical, so E. coli standing at its full height would be 20 times higher.

On the smaller end we find among bacteria the petite Mycoplasma, and among viruses the little runt, porcine circovirus (PCV). Mycoplasma measures 0.3 microns, and PCV, 0.02 microns. So Mycoplasma won't feel so small anymore when it towers over PCV at ~10 times its size. That is until PCV's big sister Mimivirus steps in. One of the larger known viruses, she's a hefty (for viruses, that is) 0.5 microns, or about one and a half times the size of Mycoplasma. A virus larger than a bacterium! Mycoplasma would surely cower off with its flagellum between its legs – that is, if it had one.

Mycoplasma has recently received a boost to its size-challenged ego, however, with the discovery of the ultra-small bacteria. Researchers examined microbes from groundwater passed through a filter with a pore size of 200 nanometers and found bacteria with sizes as small as 0.22 by 0.18 microns – close to the assumed minimal sizes of live organisms worked out by a panel of the National Research Council in 1999.

Then there are the veritable monsters, such as the gram-negative bacteria Thiomargarita namibiensis. These hulks are cocci that are typically 100 – 300 microns in diameter (that's 50 – 150 times larger than E. coli). Of course, the viruses have their own family freaks – the giant Pandoravirus and Pithiviruses, breaking the mold at 0.5 to 1.0 microns, and 1.5 microns, respectively. Okay, so not quite as large as E. coli, but more than three times the size of Mycoplasma ("little Mikey"?). Note, though, that Thiomargarita are Godzillas next to even these largest of viruses, 200 to 600 times larger.

Lastly, how do our subjects measure up to us? There are an estimated 10³¹ viruses on earth, and 10³⁰ bacteria (viruses outnumber bacteria ten to one, ha!), and the total biomass of either group is far greater than that of all animals combined, prompting the question – who are the true stage hogs?

For interested readers, many useful numbers associated with molecular biology can be found here.