PAG 7: Microbial techniques

Question 1

Explain the sterile techniques used when testing the effect of antibiotics on bacterial growth (5)

Answer 1

1. Wash your hands and disinfect the work area.
2. Keep a Bunsen burner nearby to sterilise the air and prevent airborne microorganisms from settling.
3. Pass the neck of broth bottles over the Bunsen burner flame when opening to prevent airborne contamination.
4. Only open a Petri dish enough to introduce the desired organisms.
5. Sterilise all equipment by passing it over the flame before and after use.

Question 2

Describe the method for testing the effect of antibiotics on bacterial growth (6)

Answer 2

1. Using a sterile pipette, add the same volume (0.1 cm³) of each antibiotic to separate Petri dishes containing agar. Include controls with no antibiotic.
2. Dip an inoculating loop into the bacterial broth, then spread the broth over the agar surface and close the dish.
3. Label each Petri dish and place in a warm incubator upside down to prevent condensation.
4. Incubate for the same period (e.g. one day) and then observe results.
5. Count the number of bacterial colonies and record the results in a table.
6. Calculate the mean number of colonies for each antibiotic