# mass of bacteria

Michael Witty mw132 at mole.bio.cam.ac.uk
Thu Apr 4 08:49:15 EST 2002

```On Thu, 4 Apr 2002, Graham Shepherd wrote:
> You can get an approximation by calculating the volume of the organism from
> typical dimensions (eg for E.coli assume it's a cylinder 1 micron long with
> a diameter of 0.5 micron) and assume that the density is the same as water.
> (It is greater, otherwise you couldn't spin them down - but it's probably
> not much greater. You could determine the density on a gradient if it's
> critical).
>
> A rough calculation indicates that the volume is about 0.2 cubic microns.
> That's 5,000,000,000 per cubic millimeter, or 5,000,000,000,000 per cubic
> cm. Assuming 1 gram per cubic cm, one bacterium weighs about 0.2 picogram.
>
> Viruses are much more variable in size than bacteria (the biggest viruses
> are bigger than the smallest bacteria). But you could do the same
> calculation for a specific virus.

. . . or you can do an experiment.  Grow 100ml E. coli stationary phase
culture.  Take a small sample and count the cells (microscope or serial
dilutions).  Centrifuge the rest (tared tube).  Weigh the cells.  Divide
weight by number of cells.  Depends if you want to suffer from
having to make assumptions or from experimental error!  Mike.

```