culturing microorganisms

• name the clear area that forms around a disc on an agar plate when bacteria die.

  zone of inhibitions/inhibition zones
• why are cultures incubated at a maximum temperature of 25 degrees in schools?

  harmful bacteria are more likely to grow above this temperature
• explain the aseptic techniques you would use when preparing an uncontaminated culture on an agar plate.

  sterilise the Petri dish and culture medium before use by heating them to a high temperature. sterilise the inoculating loop by passing it through a hot flame. Petri dish should be stored upside down to stop condensation droplets from falling.
• What can bacteria (and some other microorganisms) be cultured in?

  Nutrient broth solution or agar gel
• What will bacteria grown on agar plates form?

  Visible colonies on the surface of the jelly, or will spread out to give an even covering of jelly.
• When are cultures not kept above 25 degrees Celsius in a school lab?

  Harmful pathogens (which cause diseases) are more likely to grow.
• In industrial conditions why are cultures incubated at higher temperatures?

  So they can grow faster.
• Why do you need to use uncontaminated cultures?

  Contamination by unwanted microorganisms will affect results and could grow pathogens.

  For investigating the action of disinfectants and antibiotics
• How can you test the action of antibiotics (or antiseptics) on cultures of bacteria?

  1. Place paper disks soaked in different types (or concentrations) of antibiotics on an agar agar plate that has an even covering of bacteria. Leave some space between the disks.

  2. The antibiotic should diffuse (soak) into the agar jelly. Antibiotic-resistant bacteria will continue to grow on the agar around the disks, but non resistant strains will die. A clear area will be left where the bacteria have died-this is called an inhibition zone.

  3. Make sure you use a control. This is a paper disk than has not been soaked in antibiotic, instead soak it in sterile water. You can then be sure that any difference between the growth of the bacteria around the antibiotic disc is due to the antibiotic alone

  4. Leave the plate for 48 hours at 25oc

  5. The more effective he antibiotic is against the bacteria, the larger the inhibition zone will be.
• What steps can you take to make sure the cultures stay uncontaminated?

  1. The Petri dishes and culture medium must be sterilised before use, to kill any unwanted microorganisms on them

  2. If a inoculating loop is used to transfer the bacteria to the culture medium, it should be sterilised first by passing it through a flame

  3. The lid of the Petri dish should be secured with adhesive tape to prevent microorganisms from the air contaminating the culture

  4. The Petri dish should be stored upside down-to stop drops of condensation falling onto the agar surface.
• How can you compare the effectiveness of different antibiotics?

  By looking at the relative sizes of the inhibition zones, the larger the zone the more effective the antibiotic.
• What equation do you use to calculate the inhibition zone?

  Area = πr2
• Culturing microorganisms


  1. Place paper discs soaked in different types (or concentrations) of antibiotics on an agar plate

  2. Antibiotic should diffuse into the agar jelly

  3. Antibiotic resistant bacteria will continue to grow on the agar around the paper discs but non resistant strains will die

  4. A clear area will be left where the bacteria died - inhibition zones

  5. Use a control - a paper disc that has not been soaked in an antibiotic, instead use sterile waters can then see the effect of the antibiotic alone

  6. Leave the plate for 48 hrs at 25 degrees
• The most effective the antibiotic is against the bacteria, the larger the inhibition zone will be