The most common way to transfer bacteria to an agar plate involves using a sterile loop to pick up a bacterial sample and streak it across the agar surface in a specific pattern to achieve isolated colonies.
Here's a breakdown of the process, often referred to as a "streak plate" technique:
1. Prepare for Aseptic Technique:
- Sterilize your work area: Clean the surface with a disinfectant.
- Gather materials: You'll need:
- A bacterial culture (broth or colony on another plate).
- A sterile agar plate.
- A sterile inoculation loop (either disposable or a reusable loop that can be sterilized).
- A Bunsen burner (or alternative heat source for sterilization).
2. Sterilize the Inoculation Loop:
- If using a reusable loop, hold the loop in the flame of a Bunsen burner until it glows red-hot. This ensures all contaminants are destroyed. Let it cool completely before use (avoid waving it around to cool it as this can re-contaminate it). Disposable loops are pre-sterilized.
3. Obtain the Bacterial Sample:
- From a broth culture: Briefly remove the cap of the culture tube and pass the mouth of the tube through the flame quickly to sterilize it (as shown in the provided video snippet). Dip the sterilized loop into the broth culture to collect a small sample of bacteria. Flame the mouth of the tube again before replacing the cap.
- From a colony on an agar plate: Gently touch the sterile loop to a single, well-isolated colony on the plate. Avoid gouging the agar.
4. Streak the Agar Plate:
This is the crucial step for obtaining isolated colonies. A common method is the quadrant streak:
- First Quadrant: Gently streak the loop across a small section of the agar plate (about one-quarter of the plate). Avoid pressing too hard, which can damage the agar.
- Sterilize the Loop: Re-sterilize the loop by flaming it until red-hot and allow it to cool completely.
- Second Quadrant: Rotate the plate approximately 90 degrees. Touch the sterile loop to the end of the previous streak (first quadrant) and streak across a new quadrant. This dilutes the bacteria picked up from the initial streak.
- Sterilize the Loop: Re-sterilize the loop by flaming it until red-hot and allow it to cool completely.
- Third Quadrant: Rotate the plate approximately 90 degrees. Touch the sterile loop to the end of the previous streak (second quadrant) and streak across a new quadrant.
- Sterilize the Loop: Re-sterilize the loop by flaming it until red-hot and allow it to cool completely.
- Fourth Quadrant: Rotate the plate approximately 90 degrees. Touch the sterile loop to the end of the previous streak (third quadrant) and streak across the remaining quadrant, often using a zig-zag pattern.
5. Incubate the Plate:
- Close the agar plate and incubate it upside down at the appropriate temperature for the bacteria you are growing (typically 37°C for many common bacteria). Incubating upside down prevents condensation from dripping onto the agar surface and interfering with colony formation.
Expected Results:
If the streaking technique is successful, you should see dense growth in the first quadrant, followed by progressively less growth in subsequent quadrants. In the final quadrant, you should ideally see well-isolated colonies, each originating from a single bacterial cell.
Why this works:
The streaking technique dilutes the bacterial sample with each streak. By the final quadrant, you've spread the bacteria out enough that individual cells will grow into separate, distinct colonies.
