To  estimate  the  population  of  a  liquid bacterial culture, serial
   dilutions are made.

   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.
<LI> If  these grew from 1 ml of a total dilution of 10^5, the original
concentration of bacteria is:
<li> 15 x 10^5 / ml, or
+ 1.5 x 10^6 / ml.