Between bacteria and archaea (another type of single-celled microbe), there are more than 1030 individual cells. Although most are about one micrometre long (0.001mm), it’s such a colossal number that laid end-to-end, they would stretch for 10 billion light-years!


This bacterial thread would still be difficult to see, because one micrometre is about 75 times thinner than a human hair. But if you wrapped the thread around the Milky Way, it would encircle it more than 20,000 times, creating a 2cm-wide ribbon that might catch enough light to be visible to the naked eye.

Statistics like this show just how bad we are at visualising large and small amounts. Ten billion light-years is an unfathomable length, but if we packed all the bacteria into a cube (and it didn’t collapse under its own weight) it would only be around 10km on each side, which seems much more manageable.

In reality, up to 80 per cent of all the bacteria in the world are found in biofilms
on rocks, in the soil, on stagnant water and in virtually every other habitat, including your mouth and intestines. These biofilms are a few hundred bacteria thick and can contain bacteria, archaea and fungi of various species banding together in a kind of city.

You can see biofilms whenever you clean your house. The red, black or brown slime on your shower head, under the rim of the toilet or on the draining rack on the kitchen sink are all bacterial biofilms, each consisting of tens of millions of bacteria.

Read more:

Asked by: Tom Shepherd, Bristol


To submit your questions email us at (don't forget to include your name and location)


luis villazon
Luis VillazonQ&A expert

Luis trained as a zoologist, but now works as a science and technology educator. In his spare time he builds 3D-printed robots, in the hope that he will be spared when the revolution inevitably comes.