To estimate the population of a liquid bacterial culture,

A sample is then `plated`

on semisolid medium in a **petri dish**.
Each bacterium grows into one visible `colony`

,
so the original concentration can be found by multiplying the number of colonies by the **dilution factor**.
(Note: when high concentrations of cells are `plated`

, the colonies can fuse into a
"lawn"
of cells.)

Each of the plates above is diluted by a factor of 10. The dish on the right contains 15 colonies.

- If these grew from
**1 ml**of a total dilution of 10^{5}, the original concentration of bacteria is:- 15 x 10
^{5}/ ml, or - 1.5 x 10
^{6}/ ml.

- 15 x 10