How to T‐Streak (Microbiology)

Reviewed by Anne Schmidt

Last Updated: April 25, 2023

In nature, bacteria are found on practically every surface imaginable and unfortunately rarely exist in isolated populations containing a singular species of bacterium. This presents microbiologists with an interesting challenge obtaining isolated colonies of bacteria in a method that is both simple and rapid. The three sector streak or commonly referred to as the “T-Streak” is a basic, yet crucial skill used by microbiologist on a daily basis to isolate, identify, and study colonies of bacteria.

Part 1

Part 1 of 5:

Drawing the T

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Draw a line starting slightly above the center point. Continue drawing the line until you hit the back plate's edge.
This new line should be perpendicular to the original line, intersecting it at the starting point. The back plate of the petri dish should now be divided into three sectors, appearing as a large “T”.

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Label the back plate of the petri dish with relevant laboratory information such as your initials, date, time and culture source.

Part 2

Part 2 of 5:

Sterilizing the Inoculation Loop

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In order to eliminate the chance of contaminating the petri dish when inoculating the surface of the agar in the petri dish, the inoculation loop must be sterilized.

The object is to have the inoculation loop be slightly above the inner blue cone of the flame. Hold it there for several seconds.
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Remove the loop from the flame. At this point, the loop is sterile and extremely hot.
- The inoculation loop must be cooled, or else the residual heat will kill any bacteria taken from the culture.
Lift the lid of the petri dish just enough to insert the loop. Touch the outer edge of the nutrient agar. A sizzling sound should be heard upon first contact. Continue holding the inoculation loop against the surface for several seconds.
Avoid touching any other surface at this point, doing so will recontaminate the inoculation loop.

Part 3

Part 3 of 5:

Inoculating the Loop with a Broth Culture

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Open the source of your culture such as broth culture.
Do this for several seconds.

Part 4

Part 4 of 5:

Inoculating the Loop with a Plated Culture

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Part 5

Part 5 of 5:

Inoculating the Nutrient Agar

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Beginning in the top sector of the “T’, lightly drag the inoculation loop across the surface of the nutrient agar in a zigzag pattern. It is important to stay within the region in which you are currently inoculating.
Follow the method described in Sterilizing the Inoculation Loop. Remember to cool the inoculating loop on the edge of the nutrient agar that has not been contaminated with the original culture!
Lightly drag the inoculation loop across nutrient agar toward the second sector.
Lightly drag the inoculation loop across the nutrient agar in a zigzag pattern.
Follow the method described in Sterilizing the Inoculation Loop. Remember to cool the inoculating loop on the edge of the nutrient agar that has not been contaminated with the original culture!
Lightly drag the inoculation loop across the nutrient agar towards the third sector.
Lightly drag the inoculation loop across the nutrient agar in a zigzag pattern.