Culturing Microorganisms

• Culturing Microorganisms


  1. Bacteria (and some other microorganisms) are grown (cultured) in a "culture medium"

  2. The culture medium used can be a nutrient broth solution or solid agar jelly

  3. Bacteria grown on agar plates' will form visible colonies on the surface of the jelly

  4. To make an agar plate, hot agar jelly is poured into shallow round plastic dishes called Petri dishes

  5. When the jelly's cooled and set, inoculating loops (wire loops) can be used to transfer microorganisms to the culture medium

  6. Alternatively, a sterile dropping pipette and spreader can be used to get an even covering of bacteria

  7. In the lab at school, cultures of microorganisms are not kept above 25 °C

  8. In industrial conditions, cultures are incubated at higher temperatures so that they can grow a lot faster
• Uncontaminated cultures


  • The Petri dishes and culture medium must be sterilised before use (e.g. by heating to a high temperature), to kill any unwanted microorganisms that may be lurking on them
  • If an inoculating loop is used to transfer the bacteria to the culture medium, it should be sterilised first by passing it through a hot flame
  • After transferring the bacteria, the lid of the Petri dish should be lightly taped on to stop microorganisms from the air getting in
  • The Petri dish should be stored upside down-to stop drops of condensation falling onto the agar surface
• Investigating the effect of antibiotics on bacterial growth

  1. Place paper discs soaked in different types (or different concentrations) of antibiotics on an agar plate that has an even covering of bacteria

  2. The antibiotic should diffuse (snak) into the agar jelly. Antibiotic-resistant bacteria (i.e. bacteria that aren't affected by the antibiotic-see p 90) will continue to grow on the agar around the paper discs, 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 clise that has not been soaked in an antibiotic. Instead, soak it in sterile water

  4. Leave the plate for 48 hours at 25 °C

  5. The more effective the antibiotic is against the bacteria, the larger the inhibition zone will be
• Inhibition zone


  A clear area where the bacteria have died
• Calculating the sizes of the inhibition zones


  1. Measure the diameter of the inhibition zone

  2. Use the equation Area = π x r² to calculate the area of the inhibition zone

  3. Compare the areas of the inhibition zones to determine the relative effectiveness of the antibiotics
• The equation Area = π x r² is the equation for the area of a circle
• Don't open the Petri dish to measure the inhibition zones - they should be visible through the bottom of the dish
• Radius


  Half the diameter of the inhibition zone
• The equation Area = π x r² can also be used to calculate the area of a bacterial colony