Difference between pour plate and streak plate method

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The key difference between streak plate and spread plate is that the streak plate is used to isolate and purify a particular bacterial species from a mixture of bacteria while the spread plate is used to enumerate and quantify bacteria in a sample. 

Apart from the above difference, there are many other differences between streak plate and spread plate. In streak plate, inoculum introduces into the fresh medium using an inoculation loop or a cotton swab while in spread plate, inoculum draws using a sterile micro-pipette. Furthermore, in streak plate, zig-zag pattern lines are drawn on the surface of the fresh medium while in spread plate, the inoculum is spread evenly on the surface of the medium. Streak plate and Spread plate are two microbial techniques to isolate, purify and enumerate bacteria. There are differences as well as similarities between streak plate and spread plate. Further, both methods are commonly and routinely used in bacterial studies.

CONTENTS

1. Overview and Key Difference
2. What is a Streak Plate
3. What is a Spread Plate
4. Similarities Between Streak Plate and Spread Plate
5. Side by Side Comparison – Streak Plate vs Spread Plate in Tabular Form
6. Summary

What is a Streak Plate?

Streak plate is a technique that allows isolating and purify bacterial species in a sample. Therefore, it is a simple and easy method to perform in a lab. We can dilute the sample before introducing into the fresh growth medium. However, few materials are necessary to perform this technique. They are an inoculating loop or a cotton swab, solidified agar plates, bunsen burner, and a laminar air flow. A loopful of inoculum has to be taken from the sample and drawn zig-zag zag pattern lines on the surface of the fresh medium under the sterile environment (often inside a laminar air flow).

Inoculated plates are incubated at a suitable temperature. Separated bacterial colonies will grow on the lines drawn. Furthermore, interested or expected single colony can be further purified by adopting the same method until you get a purified bacterial species. Hence, this method is suitable for separating bacteria into single colonies and picking and purifying your desired bacterium.

What is a Spread Plate?

A spread plate is another microbial technique that allows enumerating bacteria in a sample, hence it facilitates the accurate quantification of bacteria. It provides information regarding the number of bacteria present in the sample. In this technique also it is necessary to dilute the sample before inoculating into fresh medium.

Moreover, micropipette and a sterilized spreader are the main materials necessary to perform this technique. A suitable aliquot (often 0.1 ml or 1 ml) is drawn out from the sample by a micropipette and transferred into fresh agar plate. Using a spreader, the inoculum spreads evenly throughout the surface and incubates at a suitable temperature. Aerobic bacteria will grow on the surface of the medium as separate colonies. Therefore, enumeration will be easy in this technique if the dilution is correct. According to the recommendation, the plates which have 30 to 300 colonies are selected for enumeration. Using an equation, we can do the correct quantification after enumeration.

What are the Similarities Between Streak Plate and Spread Plate?

• Streak plate and spread plate are two microbial techniques that we use in bacteriology.
• Both methods are in use to isolate or separate bacteria from a mixture.
• In both techniques, we can dilute the sample prior to inoculation.
• Both methods require solidified agar plates.
• Bacteria grow on the surface of the medium in both methods.
• In both techniques, aerobic bacteria grow on the surface of the medium.
• We use a Sterilized inoculating tool to introduce the inoculum into fresh medium in both techniques.
• A sterile environment is necessary to perform both techniques.

What is the Difference Between Streak Plate and Spread Plate?

Streak plate technique allows you to isolate and purify bacteria. On the other hand, spread plate technique allows you to enumerate bacteria. This is the key difference between streak plate and spread plate. Furthermore, we can use an inoculating loop or cotton swab as an inoculating tool in streak plate while we need a spreader for spread plate technique. A loopful of the sample is the quantity that draws from the sample in the first technique while 0.1 ml or 1 ml are the quantities of second technique. Inoculum introduces into a fresh medium by drawing zig-zag pattern lines in streak plate while the inoculum spreads evenly throughout the surface in the spread plate.

Streak plate and spread plate are two microbial techniques perform in bacteriology. Streak plate facilitates isolation and purification of a specific bacterium while spread plate facilitates enumeration of bacteria in a sample. Both methods are highly useful for bacterial studies, especially for aerobic bacteria. Streaks in a zig-zag pattern are done in a streak plate while the inoculum is evenly spread on the surface of the medium in spread plate. This is the difference between streak plate and spread plate.

Reference:

1.“Streaking (Microbiology).” Wikipedia, Wikimedia Foundation, 14 July 2018. Available here 
2.“Spread Plate Technique: Principle, Procedure and Results -.” Home -, 27 July 2017. Available here 

Image Courtesy:

1.’Legionella Plate 01’By CDC/James Gathany – CDC Public Health Image Library, (Public Domain) vis Commons Wikimedia
2.’33934804403’by R6, State & Private Forestry, Forest Health Protection’s photostream (Public Domain) via Flickr

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