Explain the Pour Plate Method

Isolated colonies can also be obtained by pour plate method. The method involves mixing of small volume of microbial suspension with molten nutrient agar at 45°C and pouring immediately into sterile petri plate. The microbial suspension should be diluted sufficiently to obtain separate colonies on plating. The pour plate method involves adding specified amount (0.1 ml or 1.0 ml) of the dilution to the sterile petri plate. Twenty to twenty five (20-25) ml of nutrient agar medium kept liquefied in a water bath at 45°-50° C is then added to the sterile plate and mixed with the dilution properly by gentle rotation of plate in a circular motion on the table top.

This results in uniform distribution of microorganisms. Once the agar has hardened, each cell is fixed in place and forms a distinct colony on incubation. Colonies appeared both within the nutrient agar, as well as, on the surface of agar plate. Microbial cells get fixed on solidification of agar and forms individual colonies on incubation. Colonies are present both on the agar surface and embedded in the nutrient medium. Colonies obtained at different dilutions are also highlighted in the Figure. Colonies growing on the surface can be used to inoculate fresh medium for pure cultures. The method can also be used for microbial cells enumeration in the original sample. We will practically try out this technique in the next practical. But here let us now highlight the advantages and disadvantages of this technique.