Difference Between Pour Plate and Spread Plate

The main difference between pour plate and spread plate is that the molten agar is poured on to the inoculum during the preparation of the pour plate whereas inoculum is spread on the surface of the solidified agar during the preparation of the spread plate. Inoculum refers to microorganisms, bacteria or fungi that grow in or on the nutrient agar.

Pour plate and spread plate are two techniques that quantify bacterial samples. Both require Petri dishes and nutrient agar. Generally, pour plates is the method for counting the number of colony-forming bacteria present in a liquid specimen. Pour plates also allow the identification of bacteria as aerobes, anaerobes or facultative aerobes. On the other hand, spread plates allow the isolation of specific clonal colonies.

Key Areas Covered

1. What is a Pour Plate
     – Definition, Method, Importance
2. What is a Spread Plate
     – Definition, Method, Importance
3. What are the Similarities Between Pour Plate and Spread Plate
    – Outline of Common Features
4. What is the Difference Between Pour Plate and Spread Plate
     – Comparison of Key Differences

Key Terms: Colony counting, Colony Isolation, Nutrient Agar, Petri Dish, Pour Plate, Spread Plate

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 Plate

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. The bacterial colonies prepared by the pour plate technique is shown in figure 1.

Figure 1: Bacterial Colonies

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 Plate and Spread Plate

• 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.

Difference Between Pour Plate and Spread Plate

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

Spread Plate: Does not allow the growth of microaerophiles

Conclusion

Pour plate is a technique that allows quantifying the colony-forming units of bacteria in a sample. It is prepared by pouring the molten nutrient agar on the inoculum. Spread plate helps to isolate clonal colonies. It is prepared by spreading the inoculum on the solidified agar. The main difference between pour plate and spread plate is the purpose and the preparation.

Reference:

1. “Making a Pour Plate | Nuffield Foundation.” The Truth behind SEN Statements in Mainstream Primary Schools | Nuffield Foundation, Available Here
2. “Microbiology – 004 – Spread Plate Method.” Microbiology – 007 – Carbohydrate Fermentation Test | Microbiology Undergraduate Program, Available Here

Image Courtesy:

1. “Enterobacter cloacae 01” (Public Domain) via Commons Wikimedia