Microbiology

• Growing bacterial colonies:

  1. The inoculating loop is sterilised before use, by being placed in a Bunsen burner flame, until it is red hot.
  2. The inoculating loop is then held in the air in the sterile field area, and allowed to cool.

  
• Growing bacterial colonies:

  3. The lid from the bacterial culture bottle is removed.

  4. The neck of the bottle is placed in the Bunsen burner flame.

  • This moves air out of the bottle & prevents other microorganisms from entering the bottle & contaminating the bacterial solution.

  
• Growing bacterial colonies:

  5. The sterilised inoculating loop is dipped into the flask containing the bacterial culture.

  
• Growing bacterial colonies:

  6. The neck of the bottle is flamed again and the cap is replaced.

  
• Growing bacterial colonies:

  7. The Petri dish lid is lifted slightly.

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

  9. The Petri dish lid is quickly replaced.

  
• Growing bacterial colonies:

  10. The lid of the Petri dish is secured with tape to prevent contamination.

  11. The Petri dish is then stored upside down in an incubator.

  12. In schools & colleges, the maximum temperature in the incubator is 25°C, to prevent the growth of harmful bacteria.

  
• Growing bacterial colonies:

  13. The inoculating loop is sterilised again and all work surfaces are disinfected.

  
• The variable in microbiology:

  Control variables:

  • Temperature of incubation
  • concentration of antiseptics/antibiotics
  • duration of incubation

  Independent variable:

  • Type of antiseptic or antibiotic applied

  Dependent variable:

  • Diameter of zone of inhibition
• The purpose of dividing the agar plate into three sections is to compare the effect of different antiseptics.
• It's important to wash hands with antibacterial soap before handling the filter paper discs.

  To remove any potential contaminants from the hands.
• The purpose of incubating the agar plate at 25°C for 48 hours is to allow bacteria to grow and form colonies.
• The purpose of spraying the work surface with disinfectant is to clean and remove potential contaminants.
• (i) To prevent contamination

  Step 3 was included so the bacteria are not killed.

  
• Plan:

  • Soak a filter disc in antiseptic 1
  • Repeat for antiseptic 2 & 3
  • Soak fresh filter disc in distilled water
  • Space the discs out
  • Label then incubate the petri dish

  Results:

  • Measure radius / diameter 
  • Calculate the zone of inhibition around each disc
  • Using πr2
  • Antiseptic which killed the most bacteria will have largest zone