What is Streak Plate Method ?
Streaking is a method that isolates a pure strain from a species of bacteria. A sample is taken from a colony and a microbiological culture is grown on the new plate in order for the organism to be identified properly.
It was Loeffler and Gaffky who first developed this method in Koch’s laboratory.
The procedure involves diluting bacteria by streaking the bacteria over the surface of the agar in the Petri dish. That way, an isolated colony can be obtained and grow into a number of cells. The culture is called a microbiological culture if the organism grows in the agar surface. (1, 2, and 3)
Picture 1: The image shows how a streak plate method is done.
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Picture 2: The streak plate isolation method as described on the steps mentioned above.
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What is the objective of the streak plate method?
The objective of streak plate method is to obtain isolated microbial colonies by creating areas of dilution on the agar petri plate. (3
What is the principle of the streak plate method?
The streak plate method requires the number of organisms in the inoculums be reduced. The procedure includes a dilution technique which requires spreading a loopful of culture over the agar plate surface.
This is to make sure that the individual cells fall apart on the agar medium surface so as separation of the different species takes place. This procedure is also called rapid qualitative isolation method. (2, 3, and 4)
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Picture 4: A pure bacterial isolate using the streak plate technique.
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Picture 5: The actual result of a streak plate technique.
What is the equipment needed for the streak plate method?
- Inoculation loop (6)
- Bacteria source
- Bunsen burner
- Agar plate (5)
- Paper towel
What to keep in mind when doing a streak plate method?
- Make sure you use only a small amount of inoculum.
- Make sure you streak lightly so as not to gouge the agar.
- See to it that the plate’s surface is free of droplets of condensed moisture.
- After streaking each quadrant, do not forget to flame the loop.
- The inoculum source can be a broth or solid culture, an environmental swab, a clinical specimen, or sedimented urine.
- The petri dish to be used should be a hundred millimeter in diameter. (3, 6, 7, and 8)
What is the purpose of the streak plate technique?
The purpose of the streak plate method is to produce an isolated colony of an organism on the agar plate. Isolation of the organism is a must in a mixed culture, especially if you need to thoroughly study the colony morphology of a particular organism. (4, 7)
How is the streak plate method done?
- The inoculating loop should be sterilized in the Bunsen burner by simply putting the loop into the flame. Wait for the loop to turn red indicating that it is already hot. Let it cool down for a few minutes.
- Using the inoculating loop, pick an isolated colony from the agar plate and spread on the first quadrant of the petri plate. (4, 5)
- Gently streak the inoculating loop using a back and forth motion.
- Put the loop in the flame and let it cool. Extend the streaks into the second quarter of the petri plate.
- Repeat step four and this time extend the streak into the petri plate’s third quadrant.
- Repeat step four and go back to the area you streaked in the third quadrant of the petri plate and this time extend the streak into the fourth quadrant of the petri plate.
- Flame the loop. (8, 9, and 10)
The streaked plate should be incubated for a total of 24 hours at a temperature of 37 degree Celsius. Carefully examine the colonies grown on the petri plate. The expected result is that all colonies must have the same general appearance.
If you notice that there is more than one type of colony, then you should start to streak again but this time on a separate plate so as to obtain a pure culture. (4, 5, and 6)
Streak plate method advantages and disadvantages
- The streak plate method enables you to select and work with individual colonies.
- It is the ideal method if you are doing general work with a certain type of microorganism.
- Through the streak plate method, you will end up with a genetically identical individual colony, which makes it easy for you to grab and transfer colonies for a microscopic examination.
- The streak plate method is convenient and hassle-free as you will be able to get visible individual colonies in a single petri plate for as long as you have a good streaking skill. (2, 6, 9, and 10)
- The streak plate method does not work with high volumes of organisms. It will not enable you to get a concentration count.
- It requires huge storage space and there is a possibility that your incubator cannot accommodate a large volume of petri plate.
- You will be required to prepare the agar ahead of time. The procedure can be a tedious process, especially if you don’t know the sample size before the lab work.
- Training and technique are required as various growth media have various densities to the agar. If you are not skilled enough, then you would end up tearing through the agar, especially if you apply too much pressure on the agar. A wrong streaking method can ruin your plate.
- The streak plate method can be time-consuming, especially if you are going to prepare a large sample size.
- It requires strict maintenance. The streak plate method will require constant use of the streaking loop. Eventually, it will wear out and you need to change it every once in a while. (2, 5, 7, and 10)