Pour plate Technique

A pour plate is a method of melted agar inoculation followed by petri dish incubation. A known volume (usually 0.1-1.0 ml) of culture is pipette into a sterile petri plate; melted agar medium is then added and mixed well by gently swirling the plate on the table top. Because the sample is mixed with the molten agar medium, a larger volume can be used than with the spread plate. However, with the pour plate method the organism to be counted must be able to briefly withstand the temperature of melted agar, 45^oC.

The cultures are inoculated into melted agar that has been cooled to 45^oC. The liquid medium is well mixed then poured into a petri dish (or vice versa) Colonies form within the agar matrix rather than on top as they do when streaking a plate. Pour plates are useful for quantifying microorganisms that grow in solid medium. Because the “pour plate” embeds colonies in agar it can supply a sufficiently oxygen deficient environment that it can allow the growth and quantification of microaerophiles.

Spread Plate Technique
Spread plate technique is an additional method of quantifying microorganisms on solid medium. With the spread plate method, a volume of an appropriately diluted culture usually no greater than 0.1 ml is spread over the surface of an agar plate using a sterile glass spreader. The plate is than incubated until the colonies appear, and the number of colonies counted. Instead of embedding microorganisms into agar, as is done with the pour plate method, liquid cultures are spread on the agar surface.

An advantage of spreading a plate over the pour plate method is that cultures are never exposed to 45 ^oC (i.e. melted agar temperatures).
Note: Surface of the plate must be dry, so that the liquid that is spread soaks in. volume greater than 0.1ml are rarely used because the excess liquid does not soak in and may cause the colonies to coalesce as they from, making them difficult to count.