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Preparing Plates
Scientists heat bottles or tubes of agar to sterilize and melt it. They cool the agar in baths of water, for ease of handling. While the agar is still liquid, at around 50 C or 122 F, scientists pour it into a sterile plate or "petri dish," aiming to produce a smooth, even surface with no bubbles.
Streaking Plates
To isolate bacteria for a "pure culture," scientists dip a sterile loop of wire into the bacteria source. They gently draw the loop over the agar, making four or five streaks. They sterilize the loop in a flame. After the loop has cooled for 10 seconds, they draw it along the initial streaks, picking up a few bacteria and depositing them in another set of streaks on the agar. Successive streaks dilute the bacteria further, forming ever-more-isolated samples.
Inverting Plates
The plate is heat-treated to sterilize it, before the agar is added. However, droplets of water from the surrounding air will condense on the cooling walls of the plate. These droplets could contaminate the agar if they come into contact with it and carry bacteria from one streak to another. To prevent this, scientists invert the plate, so any condensation drains away from the agar surface.
When to Invert?
The University of Missouri advises its microbiology students to invert an agar plate as soon as the medium has solidified and before the walls of the plate have cooled enough for condensation to form. Students should also invert and dry the plate's lid and store dry plates upside down in a refrigerator until needed.