How to streak agar plates

Written by samuel sohlden
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How to streak agar plates
Properly streaking an agar plate is an essential method in microbiology. (Bacteria Colonies image by ggw from Fotolia.com)

Properly streaking an agar plate is an essential method in microbiology. Agar plates are used to grow bacteria and fungi. When streaked with the proper technique, it's possible to isolate species colonies from a mixture of organisms, create individual colonies necessary for experimentation and illustrate microbiological activity from a sample. Once this simple technique has been learnt, it's possible to perform a large array of microbiological experiments.

Skill level:
Easy

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Things you need

  • Agar plate
  • Inoculating loop
  • Bacterial culture
  • Alcohol candle

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Instructions

  1. 1

    Find a culture to spread on an agar plate. Cultures are typically stored frozen, but a liquid suspension or a bacterial colony from an agar plate can be used as a substitute.

  2. 2

    Light the alcohol candle. Sterilise the inoculating loop in the open flame by heating the entire part of the tool that will come in contact with the plate until it is glowing red.

  3. 3

    Dip the loop portion of the inoculation loop into the culture after it has cooled. If a bacterial culture is not available, skim the top of an individual colony on an agar plate.

  4. 4

    Open the lid on the agar plate. Spread the bacteria or fungi onto one quarter of the plate in a zigzag pattern. Flame the inoculation loop.

  5. 5

    Spread the sample you just spread on the plate by starting at the edge of the zigzag pattern and repeating the procedure. This streak should occupy another quarter of the plate. The purpose of this method is to reduce the bacterial sample to a small enough size for individual colonies to grow.

  6. 6

    Repeat the previous step, smearing sample from the second quarter of the plate to the third quarter after again sterilising the inoculation loop.

  7. 7

    Sterilise the loop and smear sample from the third quarter of the plate into the final fourth quarter.

  8. 8

    Put the lid on the agar plate and place it in an incubator or allow it to incubate at room temperature.

Tips and warnings

  • All smearing patterns are in a zigzag pattern. The purpose of this pattern is to reduce the sample to a small enough size to produce colonies.
  • Flame sterilise between making each zigzag pattern to eliminate excess sample, making it easier to produce individual colonies after incubation.
  • Certain bacteria or fungi can be pathogenic. Always autoclave agar plates after streaking and incubation to eliminate this threat.
  • The flame produced by the alcohol candle can cause burns or even start a fire. Use appropriate caution when using the alcohol candle and always keep a fire extinguisher nearby should it fall and break while lit.

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