Streak plate method and pure cultures

You: Dr. Spencer, now that we've quantified the bacterial load, what's our next step? 

Dr. Spencer: We need to isolate pure cultures for further tests. It's crucial because we need to ensure that the bacteria we're studying are not mixed with other types. This way, we can accurately identify the pathogen responsible for the contamination. When you work with potentially mixed cultures, like our initial samples, there's a risk of interference from other bacteria, which can skew our results or lead to misidentification.

Sam: Don’t we need to perform the streak plate method to isolate pure cultures? 

You: That’s right, I remember we learnt about this in class. We’d better practice though before we use the actual samples. You proceed to practice using the simulation tool online: 

https://webcampus.med.drexel.edu/simulation/microbiology/lab1/ and also watch a video.

Confident that you remember how to perform the streak plate method, you proceed to take out the material needed and perform the technique following the steps below.

Material 

• LB agar plates

• Bunsen burners

• Inoculating loops

• 200mL pipette with sterile tips

• Markers

Procedure

• Label the bottom of the agar plate with relevant information, including the date and the identity of the specimen.

• Sterilize the inoculating loop by passing it through the Bunsen burner flame until it is red-hot. Allow it to cool briefly.

• Dip the cooled inoculating loop into the sample containing the mixed culture.

• Gently streak the loop across one quadrant (approximately one-fourth) of the agar plate surface, creating parallel lines or a zigzag pattern.

• Flame the loop again to sterilize it, allowing it to cool after the flame.

• To inoculate the second sector, rotate the plate about 90 degrees. Overlap the first streaked sector slightly, then streak into a second sterile area of the plate using the loop.

• Create another set of streaks within this new sector.

• Repeat the process for remaining sectors. Sterilize and cool the loop once more. Rotate the plate again and streak into a third section, overlapping the previous one slightly.

• Continue this process to create a final fourth set of streaks, ensuring the loop is sterilized between each set.

• Close the agar plate, invert it and incubate it at 37°C overnight