Quadrant streaking is an integral step in the isolation of specific species of bacteria. Isolating specific bacteria is necessary if a researcher wants to analyze a specimen that they have. If a specimen is not isolated, then any further tests on that specimen would be contaminated and the results would not be able to be used. Below is a visualization of the quadrant streak method.
Now that I've explained why someone would want to do quadrant streaking, I will explain how to do it! From a master agar plate, select the specimen that you would like to isolate. Label a new agar plate with a specimen ID, such as 01, and the date. Next, flame a metal inoculating loop that will collect and transfer the specimen from the master plate to the new plate and let it cool for 20 seconds. The flaming sterilizes the loop so there are no contaminants from anything that may be on the loop. Do not touch the loop, or it will no longer be sterile and will need to be flamed and cooled again.
Once the loop has cooled, take a small sample of the specimen from the master plate and make a back-and-forth motion without overlapping in a quadrant of the new plate. Flame the loop and let it cool after each quadrant.
Then for the second quadrant, make another back-and-forth motion starting from one of the lines of the back-and-forth motion on the first quadrant. This takes a small amount of the initial sample.
For the third quadrant, make a back-and-forth motion starting from the second quadrant without overlapping.
For the fourth quadrant, make a back-and-forth motion from the last line of the third quadrant without any overlap. This new plate will be incubated for 24 to 48 hours and the result should show that the fourth quadrant has small isolated colonies without contamination.
When I was working with my isolates this semester, this step was really important in selecting my antibiotic-producing specimen. I found that my fourth specimen was an antibiotic producer, so I did quadrant streaking, but something happened that I was not expecting. When I saw the incubated quadrant streak plate, there were two different colored bacterial isolates from what I thought was just one specimen! If I hadn't quadrant streaked, then I would not have known that there were two specimens in my fourth isolate, which I separated into 04A (left) and 04B (right) for further testing.
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