When studying microbiology, one of the most important techniques you will learn is how to streak an agar or blood plate. There are two types of streaking, one for growing new cultures and one for isolating individual colonies. In addition, there are two types of streaking methods for isolation.
Petri dishes are a nutrient rich environment, so it is important to use sterile techniques at all times. Never leave a Petri dish open to the air. Always fire your loop before and after transferring the bacteria from one medium to the next. Some labs may require wearing a mask or working inside a downdraft hood to minimize environmental contaminants.
Basic Streaking Technique
Heat the inoculating loop and fire until orange to remove any microorganisms. Allow the loop to cool for 20 or 30 seconds. Do not blow on the loop --- you will reintroduce microorganisms. Collect a sample of bacterial culture from either a broth or a Petri dish. You do not need a visible bacterial presence. Lift the lid of the Petri dish to be streaked at a 45-degree angle, place the loop at the back of the dish, touch the loop to the medium and swipe the loop back and forth across the surface of the dish in an S-shape from the back to the front. Some labs may want you to turn the dish 90-degrees and repeat the inoculating procedure for a full coverage.
Streaking for Isolation
Streaking for isolation involves a single inoculation of a section of the Petri dish and then thinning the culture by dragging microorganisms from the initial section to two or three additional sections, effectively thinning the microorganism population. Commonly used when you need to separate a mixed culture of bacteria, bacteriologists also use this method to isolate a line of bacteria born from a single progenitor.
To accurately perform a T-streak, draw a "T" on the back of the Petri dish. Flip the dish lid down and, with a permanent marker, divide the dish in half. Then draw a line dividing one of the halves in half to form a "T." Turn the dish so that the largest section is on your right. Inoculate the loop according to the above procedure. Lift the lid and inoculate the medium with the S-swipe, but do not cross the half line. Re-heat the loop to kill any microorganisms. Turn the dish to the left until the first of the smaller sections is at the bottom right-hand corner. Start your S-swipe in the right-hand corner of the first section you streaked and streak through the new section. Fire the loop to remove any microorganisms. Turn the plate one more time and repeat the S-swipe from the second section through the third.
This is also a method for streaking for isolation but uses four sections instead of three. Divide the bottom of the Petri dish into four equal parts with a permanent marker. Use the basic streaking procedures to inoculate the first quadrant. Work in a counter-clockwise fashion, pulling the microorganisms from one quadrant to the next as you did with the T-streak and firing the loop before inoculating the next quadrant. Fire the loop and S-swipe from the first through the adjacent quadrant.
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