PAG 7: Microbial Techniques Flashcards
What sterile techniques should be used when investigating the effect of antibiotics on bacterial growth? (5)
Wash hands and disinfect the work area
Use a Bunsen burner nearby to sterilise the air and prevent airborne contamination
Pass the neck of the broth bottle over the Bunsen burner flame to prevent microorganisms in the air from entering
Open Petri dishes only slightly when introducing bacteria to minimise contamination
Sterilise all equipment by passing it over the flame before and after use
Describe the method for investigating the effect of antibiotics on bacterial growth. (6)
- Using a sterile pipette, add 0.1 cm³ of each antibiotic to different Petri dishes
- Dip an inoculating loop into the bacterial broth and recap the broth bottle
- Streak the broth over the agar surface, close the Petri dish, and tape it shut
- Label each Petri dish
- Place Petri dishes in a warm incubator upside down to prevent condensation from dripping onto the agar
- Leave plates for a set time (e.g., 24 hours), then observe bacterial growth
Why should Petri dishes be incubated upside down? (1)
To stop condensation from forming on the lid and dripping onto the agar, which could affect bacterial growth
Why are two Petri dishes with no antibiotics and two uncultured Petri dishes included in the experiment? (1)
To act as controls for comparison to ensure that any changes in bacterial growth are due to the antibiotic and not other factors
How is bacterial growth measured in this experiment? (1)
By counting the number of bacterial colonies formed on each plate.
Why should plates be left for the same amount of time before observation? (1)
To ensure a fair test by keeping bacterial growth conditions consistent across all plates.
How is the effect of antibiotics quantified in this experiment? (1)
By calculating the mean number of bacterial colonies formed for each antibiotic
What should be done if colonies overlap and make counting difficult? (1)
Perform serial dilutions to reduce the number of bacteria before plating, making colony counting easier
Explain why the same volume of culture was added to each agar plate. (1)
So the same number of bacteria are transferred to allow comparison.
Explain why the size of the clear zone is a measure of the antimicrobial properties of a substance. (2)
Clear zone is where bacteria cannot grow
So the larger the area of the clear zone, the more effective the antimicrobial properties of the substance
