What is a Pour Plate?
Pour plate refers to a plate prepared by mixing the inoculum with cooled but, still molten medium before pouring the latter into the Petri dish. It is the method of choice for counting the number of colony-forming bacteria present in a liquid specimen. A fixed amount of inoculum from sample is placed in the center of a sterile Petri dish and then the cooled, molten agar is poured on to the dish and mixed well. The plate can be inverted and incubated after solidification.
In the pour plate method, bacteria grow both on the surface as well as within the medium. Small colonies appear within the medium due to lack of oxygen. One can count each colony in the plate as each colony represents a colony-forming unit (CFU).
What is Spread Plates
Spread plate is a technique that counts or isolates bacterial colonies on the surface of the agar. A small amount of inoculum can be poured on to the solidified agar and spread with the use of a spreader. Here, the spreader should not be too hot, as it may kill the bacteria in the sample.
Bacterial growth of the spread plate only occurs on the surface of the plate. Therefore, spread plate technique gives well-separated colonies that are easy to count and pick up. The dilution of the sample is a critical factor in obtaining well-separated colonies.
Similarities Between pour and spread plate method
Pour plate and spread plate are two techniques used to grow bacteria in order to quantify them.
The preparation of both types of plates requires nutrient agar and Petri dishes.
After inoculation, both plates are incubated for bacterial growth.
Comparison Summary
Definition
Pour Plate: A plate prepared by mixing the inoculum with the cooled but still molten medium before pouring the latter into the Petri dish.
Spread Plate: A technique used to count or isolate bacterial colonies on the surface of the agar
Preparation
Pour Plate: Molten agar is poured on the inoculum in a Petri dish and gently swirled
Spread Plate: Inoculum is spread on the solidified agar on a plate by a spreader
Amount of Inoculum
Pour Plate: 1 ml
Spread Plate: 0.1 ml
Colony Growth
Pour Plate: In and on the medium
Spread Plate: Only on the surface of the medium
Area of Growth
Pour Plate: More area to grow
Spread Plate: Less area to grow
Purpose
Pour Plate: To count the number of colony-forming bacteria in a sample.
Spread Plate: To isolate specific clonal colonies.
Advantages
Pour Plate: Allows the identification of bacteria as aerobes, anaerobes or facultative aerobes; allows the growth of microaerophiles
Spread Plate: Even distribution of colonies
Disadvantages
Pour Plate:
- Picking a colony may interrupt other colonies.
- Loss of viability of heat-sensitive organisms coming into contact with hot agar.
- Embedded colonies are much smaller than those which happen to be on the surface. Thus, one must be careful to score these so that none are overlooked.
- Reduced growth rate of obligate aerobes in the depth of the agar.
Spread Plate:
- Does not allow the growth of microaerophiles.
- Only a small inoculation volume can be used.
- There is a Higher Probability of Contamination prior to isolation.
- Can be used for qualitative and not quantitative studies because it cannot be used for the enumeration of the approximate number of bacteria in the given sample.
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