Culturing microorganisms

• Bacteria (and some other microorganisms)


  Grown (cultured) in a "culture medium"
• Culture medium


  Contains the carbohydrates, minerals, proteins and vitamins they need to grow
• Types of culture medium

  • Nutrient broth solution
  • Solid agar jelly
• Growing bacteria on agar plates

  1. Pour hot agar jelly into Petri dish

  2. Jelly cools and sets

  3. Use inoculating loop to transfer microorganisms

  4. Alternatively, use sterile pipette and spreader

  5. Microorganisms multiply
• In the lab at school, cultures of microorganisms are not kept above 25°C, because harmful pathogens are more likely to grow above this temperature
• In industrial conditions, cultures are incubated at higher temperatures so that they can grow a lot faster
• Investigate the effect of antibiotics on bacterial growth

  Antibiotics -  are substances which kill or reduce the growth of bacteria.

  Examples: 

  • Penicillin
  • Streptomycin
• How you can test the action of antibiotics on cultures of bacteria

  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. Leave some space between the discs

  3. The antibiotic should diffuse (soak) into the agar jelly

  4. Antibiotic-resistant bacteria (i.e. bacteria that aren't affected by the antibiotic) will continue to grow on the agar around the paper discs, but non-resistant strains will die

  5. A clear zone will be left where the bacteria have died
• Control

  A paper disc that has not been soaked in an antibiotic, instead soaked in sterile water
• Any difference between the growth of the bacteria around the control disc and around one of the antibiotic discs is due to the effect of the antibiotic alone (and not, e.g. something in the paper)
• Leave the plate for 48 hours at 25°C
• Effectiveness of antibiotic


  The more effective the antibiotic is against the bacteria, the larger the clear zone will be
• Aseptic techniques


  Techniques used to prevent contamination of cultures by unwanted microorganisms
• Aseptic techniques are used to prevent contamination of cultures by unwanted microorganisms
• Contamination would affect your results and could potentially result in the growth of pathogens
• Avoiding contamination

  1. Sterilise Petri dishes and culture medium before use (e.g. by heating to a high temperature) to kill any unwanted microorganisms

  2. Sterilise inoculating loop by carefully passing it through a hot flame before use

  3. Work near a Bunsen flame (hot air rises, so any microorganisms in the air should be drawn away from your culture)

  4. Lightly tape the lid of the Petri dish after transferring the bacteria (to stop microorganisms from the air getting in)

  5. Store the Petri dish upside down (to stop drops of condensation falling onto the agar surface)
• Calculate the sizes of the clear zones to compare results:

  1. Compare the effectiveness of different antibiotics on bacteria by looking at the relative sizes of the clear zone.

  • The larger the clear zone around the disc, the more effective the antibiotic is against the bacteria. 

  1. To calculate the area of a clear zone, you need to use this equation: Area = r2

  • You're likely to use the units cm² or mm².
  • Divide the diameter of zone A by two to find the radius. 
  • Stick the radius value into the equation area = r²
  • Repeat steps 1 and 2 for zone B.
  • Compare the sizes of the areas.