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 10⁵, the original concentration of bacteria is:
  • 15 x 10⁵ / ml, or
  • 1.5 x 10⁶ / ml.