How To Streak a Bacterial Culture

Bacterial culture streaking allows bacteria to reproduce on a culture medium in a controlled environment. The process involves spreading bacteria across an agar plate and allowing them to incubate at a certain temperature for a period of time. Bacterial streaking can be used to identify and isolate pure bacterial colonies from a mixed population. Microbiologists use bacterial and other microbial culture streaking methods to identify microorganisms and to diagnose infection.

What You Need:

• Culture plate with microorganisms
• Inoculating loop or sterile toothpicks
• Agar plates
• Bunsen burner or another flame producing instrument
• Gloves
• Tape

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Gram Stain Procedure in Microbiology

By Anne Marie Helmenstine, Ph.D.

Here's How:

1. While wearing gloves, sterilize an inoculating loop by placing it at an angle over a flame. The loop should turn orange before you remove it from the flame. A sterile toothpick may be substituted for the inoculating loop. Do not place toothpicks over a flame.
2. Remove the lid from a culture plate containing the desired microorganism.
3. Cool the inoculating loop by stabbing it into the agar in a spot that does not contain a bacterial colony.
4. Pick a colony and scrape off a little of the bacteria using the loop. Be sure to close the lid.
5. Using a new agar plate, lift the lid just enough to insert the loop.
6. Streak the loop containing the bacteria at the top end of the agar plate moving in a zig-zag horizontal pattern until 1/3 of the plate is covered.
7. Sterilize the loop again in the flame and cool it at the edge of the agar away from the bacteria in the plate that you just streaked.
8. Rotate the plate about 60 degrees and spread the bacteria from the end of the first streak into a second area using the same motion in step 6.
9. Sterilize the loop again using the procedure in step 7.
10. Rotate the plate about 60 degrees and spread the bacteria from the end of the second streak into a new area in the same pattern.
11. Sterilize the loop again.
12. Replace the lid and secure with tape. Invert the plate and incubate overnight at 37 degrees Celsius (98.6 degrees Fahrenheit).
13. You should see bacterial cells growing along the streaks and in isolated areas.

Tips:

1. When sterilizing the inoculating loop, make sure that the entire loop turns orange before using on the agar plates.
2. When streaking the agar with the loop, be sure to keep the loop horizontal and only streak the surface of the agar.
3. If using sterile toothpicks, use a new toothpick when performing each new streak. Throw all used toothpicks away.

Safety:

When growing bacterial colonies, you will be dealing with millions of bacteria. It is important that you follow all lab safety rules. Precautions should be taken to ensure that you don't inhale, ingest, or allow these germs to touch your skin. Bacterial plates should be kept closed and secured with tape while incubating. Any unwanted bacterial plates should be disposed of properly by placing them in an autoclave to kill the bacteria before discarding them. Household bleach may also be poured over the bacterial colonies to destroy them.