Contamination and Avoiding Contamination

• Contamination

  When growing cultures of bacteria, we provide conditions that are good for lots of different types of bacteria to grow. If bacteria that are not being investigated grow, then our investigation has been contaminated.
• Sources

  • Potential sources of contamination are:
  • Skin
  • Air
  • Soil
  • Water
• Prevention

  • For investigations into antibiotics to be useful, the cultures must be pure (do not contain bacteria that are not being investigated).
• Aseptic Technique

  To prevent contamination (from skin, air, soil, or water), aseptic (free from contamination) techniques are used on all apparatus. These techniques kill and prevents the entry of unwanted bacteria. 
• one way to prevent contamination is Temperature

  • In schools, the maximum temperature at which cultures should be incubated (stored) is 25 degrees Celsius.
  • This extra precaution reduces the risk of harmful bacteria growing.
• one way to prevent contamination is Boiling

  Solutions and agar must be boiled for sterilisation.
• one way to prevent contamination is Flames

  Inoculation loops (tools used to transfer bacteria) must be passed through a flame for sterilisation (removes bacteria).
• one way to prevent contamination is Lids

  • To stop air from getting into the Petri dish, the lid must be rapidly taken off and put back on when using the inoculating loop to add bacteria to the agar.
  • This lid should be taped on and the dish should be stored upside down to prevent condensation forming on the lid and then dripping onto the agar.