Serial dilutions and pour plate technique

Serial dilution involves repeatedly mixing known amounts of source culture with (sterilised) liquid. 1 ml added to 9 ml gives a 10-fold dilution; 1 ml added to 99ml gives a 100-fold dilution. When fixed amounts of this dilution series are mixed with an appropriate agar and incubated, then different numbers of colonies will be obtained. By working back from an easily counted plate and using the appropriate dilution factor, the number of micro-organisms in the original source culture can be calculated.

In a pour plate technique, a small amount of inoculum from a broth culture is added by pipette to the centre of a Petri dish. Cooled, but still molten, agar medium in a test tube or bottle is then poured into the Petri dish. The dish is then rotated gently, or moved back and forth, to ensure that the culture and medium are thoroughly mixed and the medium covers the plate evenly.  Pour plates allow micro-organisms to grow both on the surface and within the medium. Most of the colonies grow within the medium and are small in size and may be confluent. The few colonies that grow on the surface are of the same size and appearance as those on a streak plate.