Uses for a Spread Plate

Updated February 21, 2017

A spread plate is a valuable tool to a microbiologist because it is very versatile and easy to use. A spread plate is different from a drop or pour plate and has many advantages over them. The basic process of creating a spread plate requires spreading the bacteria over the agar in order to separate the bacteria.

Isolating Bacterial Colonies

The spread plate method is designed to isolate bacterial colonies, because as the inoculating tool passes over the gel, it deposits bacteria along the way. At the end of the spreading, the inoculating tool should pass over a patch of agar that it has not previously streaked. Agar is simply a scientific term used to refer to the bottom of a petri dish where you have a growth. Spreading over the agar allows the most separated bacteria to settle on a relatively empty space. The result of this is that the bacteria are far apart enough to grow into their own colonies. This makes the spread plate a powerful tool for creating pure colonies out of an originally blended mixture.

Determining the Concentration of the Original Mixture

The spread plate is also useful for determining the concentration of the original mixture. You can use a spread plate to do this by predetermining the size of the aliquot from the original mixture as well as a dilution factor for the spread. The aliquot is simply a portion or part of a whole. When the spread plate has been made and the colonies have been counted, it is possible for you to calculate the original concentration using the previously described numbers because with the spread technique, every colony should come from a single bacterium.

Isolating Organisms

Isolating organisms is another use for a spread plate. This is only possible if the colonies are visibly different when they grow on the plate. Even if one of the two organisms outnumbers the other by a thousandfold, it would be possible to isolate the other organism. After creating the spread plates and watching the colonies grow, you need to create new plates using the isolated organism of interest. It would, however, likely take dozens of plates to ensure that the organism is properly isolated.

Displaying Differences in Colonies

The spread plate allows for great visualisation of the bacterial colonies after they are grown. This is possible because the separated colonies are isolated enough to grow purely without another colony interfering. Separating the colonies on the spread plate allows you to easily view the differences in between colonies in order to study them. This is useful with bacteria that are uncommon or hard to grow.

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Alexis Writing has many years of freelance writing experience. She has written for a variety of online destinations, including She holds a Bachelor of Arts in communication from the University of Rochester.