Fundamentals of Microbiology

2.5.8 Plating techniques

In plating techniques, mixture of cells is diluted and spread out on an agar surface or mixed with agar medium in a petridish, so that every cell grows into a completely separate colony (macroscopically visible growth or cluster of microorganisms on a solid medium). There are two common plating techniques followed for enumeration of bacteria and fungi.

1. Spread plate
2. Pour plate

In spread plating, 0.1 or 0.5 ml of diluted samples are spread out on an agar surface so that every cell grows into a colony which are counted and the load in original sample estimated.

In pour plating, the original sample is diluted several times and small volumes (1 ml) of diluted samples are mixed with liquid agar that has been cooled to about 45^oC and the mixtures are poured into sterile culture dishes. After the agar has solidified, each cell is fixed in place and forms an individual colony. The total No. of colonies equals the number of viable microorganisms in the diluted sample.

The results of plating technique are always expressed as colony forming units rather than the no. of microorganisms. Low counts will result if clumps of cells are not broken up and dispersed, and it is not certain that each colony arose from an individual cell.