Investigating Microbial Growth (Required Practical)

• The effect of antimicrobial substances on the growth of microorganisms can be investigated using the following steps:

  • Make conditions aseptic
  • Spread bacteria
  • Place the antibiotic ring
  • Incubate
  • Observe growth patterns
• It is important to make sure the conditions for the experiment are aseptic, this prevents the growth of unwanted microorganisms that might affect the results
• Why are aseptic techniques important?

  Prevents the growth of unwanted microorganisms that might affect the results
• What are some examples of aseptic techniques?


  Disinfecting surfaces, sterilising equipments, flaming the neck of bottles upon opening
• To spread the bacteria, use a pure culture of the bacteria to make a lawn spread on the petri dish
• Place an antibiotic ring on the surface of the agar and place the lid of the petri dish on and secure loosely with tape
• Why shouldn’t the lid of a petri dish be sealed shut?

  Aerobic bacteria need oxygen to grow.
• Do not completely tape shut the lid of the Petri dish, to allow the flow of oxygen and prevent the growth of anaerobic bacteria
• Incubate the Petri dish upside down for 24-48 hours to allow the bacteria to grow
• How long should the petri dish be incubated?

  24-48 hours
• Incubating the Petri dish upside down prevents condensation from dripping onto the agar
• Why is the petri dish stored upside down?

  To prevent condensation from dripping onto the agar
• A zone of inhibition should be seen around the antibiotic ring
• What is a zone of inhibition?


  Area around an antibiotic disk where bacteria do not grow.