Aseptic Techniques (WJEC GCSE Biology)

• The study of microorganism growth may involve the following:
  • Inoculation
    • Inoculation is the purposeful transfer of bacteria into a sterile growth medium
    • E.g. nutrient agar in a Petri dish can inoculated with bacteria or fungi
  • Plating
    • Inoculation of nutrient agar in a petri dish is known as plating
      • E.g. bacteria samples are spread out over the surface of an agar plate using an inoculating loop
      • This separates microorganisms and allows individual bacteria colonies to be observed
  • Incubation
    • Following plating the inoculated agar plates can be incubated at a set temperature for a specified time period
    • Schools and laboratories will have different rules and regulations as to the temperatures that can be used for incubation
• During all of the processes described above it is essential that aseptic techniques are used; this prevents:
  • Release of infectious microorganisms into the environment
  • Contamination of cultures with unwanted microorganisms

Examples of aseptic technique

• Sterilise Petri dishes
  • This involves heating equipment to a high temperature to kill microorganisms 
• Sterilise nutrient agar
  • This should be carried out before the agar is poured into the sterile Petri dishes
• Disinfect work surfaces with disinfectant or alcohol before inoculation
• Work next to a lit Bunsen burner
  • This creates convection currents that stop contaminants from falling onto the growth media
• Use flamed inoculating loops or sterile swabs when transferring cultures 
  • Heating loops until red-hot kills any unwanted microorganisms
  • This should be done both before and after inoculation
• Keep the lid on Petri dishes when not in use, and lift the lid at an angle during inoculation
  • This reduced contamination risk from airborne microorganisms
• Tape the Petri dish lids closed after plating
  • Loss of Petri dish lids could lead to the escape of potentially harmful microorganisms
• Incubate plates at a safe temperature of 25 °C
  • This prevents the growth of pathogenic microorganisms which are more likely to grow at human body temperature
• Sterilise or dispose of all equipment after use

Inoculation diagram