culturing microorganisms

• Culture medium


  Contains carbohydrates, minerals, proteins and vitamins to grow microorganisms
• Culturing microorganisms


  Growing many microorganisms in the lab using nutrients
• Reasons for sterilisation steps


  • Prevents contamination with other microorganisms
  • Prevents competition for nutrients and space
  • Prevents introduction of harmful microorganisms
• Reasons for other culturing steps


  • Inoculating loops sterilised to kill unwanted microorganisms
  • Petri dish lid sealed but not completely to allow oxygen
  • Petri dish stored upside down to prevent condensation
  • Incubated at 25°C to prevent growth of harmful bacteria
• Growing bacterial colonies: inoculation


  The process of transferring bacteria from a broth to an agar plate
• Inoculation process


  1. Inoculating loop sterilised by placing in Bunsen burner flame until red hot

  2. Inoculating loop held in air in sterile field area and allowed to cool

  3. Lid removed from bacterial culture bottle

  4. Neck of bottle placed in Bunsen burner flame to move air out and prevent contamination

  5. Sterilised inoculating loop dipped into flask containing bacterial culture

  6. Neck of bottle flamed again and cap replaced

  7. Petri dish lid lifted slightly

  8. Zig-zag streaks made gently and carefully across agar with inoculating loop

  9. Petri dish lid quickly replaced

  10. Petri dish lid secured to prevent contamination

  11. Petri dish stored upside down in incubator

  12. Maximum incubator temperature 25°C to prevent growth of harmful bacteria

  13. Inoculating loop sterilised again and all work surfaces disinfected
• By taking filter paper discs, soaking them in different antiseptic or antibiotic solutions, and then placing them on an agar plate, scientists can find out how good the different solutions are at killing bacteria
• Zones of inhibition

  • Bacteria that are not affected by the substance will grow - bacterial growth will be visible close to the paper discs.
  • Bacteria that are affected by the substance will die - leaving a clear area surrounding the paper discs
  • Clear area is inhibition zone
• inhibition zone can be calculated using the equation:

  area = π r2
• A larger zone of inhibition indicates the substance is more effective  at killing the bacteria.