Culturing Micro-Organisms

• What is the purpose of culturing microorganisms in the lab?

  • To study microorganisms
  • To grow many of them using nutrients
• What does the culture medium for microorganisms contain?

  Carbohydrates, minerals, proteins, and vitamins
• What are the two methods to grow microorganisms in the lab?

  1. In nutrient broth solution
  2. On an agar gel plate
• How is a nutrient broth solution prepared for culturing bacteria?

  A suspension of bacteria is mixed with sterile nutrient broth and shaken
• What is the role of cotton wool in the culturing process?

  It prevents air from contaminating the culture
• Why is shaking the flask important during culturing?

  It provides oxygen for the growing bacteria
• What are the steps to make an agar gel plate?

  1. Pour hot sterilised agar jelly into a sterilised Petri dish
  2. Allow it to cool and set
  3. Dip inoculating loops in the microorganism solution and spread over agar
  4. Tape the lid and incubate upside down
• Why must Petri dishes and culture media be sterilised before use?

  To prevent contamination with other microorganisms
• What is the purpose of sterilising inoculating loops?

  To kill unwanted microorganisms
• Why should the lid of the Petri dish be sealed but not completely?

  To prevent contamination while allowing oxygen in
• Why should the Petri dish be stored upside down?

  To prevent condensation from disrupting growth
• What is the optimal incubation temperature for culturing bacteria?

  25 degrees Celsius
• Why is a higher incubation temperature risky for culturing bacteria?

  It allows harmful bacteria to grow
• How do bacteria multiply in suitable conditions?

  By binary fission
• If the mean division time for a bacterium is 20 minutes, how many divisions occur in 1 hour?

  3 divisions
• What is the formula to calculate the number of bacteria at the end of the growth period?

  Bacteria at beginning x 2 number of divisions = bacteria at end
• What does the variable 'n' represent in the bacteria growth formula?

  The number of divisions
• How can bacteria be tested for antibiotic effectiveness?

  1. Soak paper discs in antibiotics and place on agar with bacteria
  2. Measure the zone of inhibition around the discs
  3. A control disc soaked in sterile water shows no bacterial death
• What does a larger zone of inhibition indicate?

  More bacteria are killed, indicating a more effective antibiotic
• At what temperature should the antibiotic testing plates be left for incubation?

  25 degrees Celsius
• What formula is used to calculate the cross-sectional area of colonies or inhibition zones?

  Area = π x radius squared
• How do you calculate the number of divisions of bacteria?


  Divide the time the population is left for by the mean division time for that bacteria
• What does a bigger inhibition zone indicate?


  Greater effectiveness at killing bacteria and more effective the antibiotic is