24. Investigation into the numbers of bacteria in milk

  • 00:43 Why must the fermented milk be diluted before plating?
  • 01:50 Why is the Petri dish taped?
  • 01:58 Why is the culture incubated at 25oC?
  • 02:00 What is the assumption that is made about the number of colonies counted?
  • 02:14 What is the total dilution factor by which you multiply the number of colonies counted to get the number of bacteria in 1cm3 original fermented milk?
  • preparing dilutions

  • 2 samples of fermented milk e.g. Yakult or Actimel one with a distant use-by date and one at its use-by date

  • 1cmsyringes 

  • 10cmsyringe

  • distilled water

  • screw-cap bottles e.g. universal bottles

  • plating and growing samples

  • 9cm petri dishes

  • molten MRS agar, maintained at approximately 50oC

  • sticky tape

  • marker pen

  • 25oC incubator


Hazard Risk Control measure

Growing microbes – contamination with other microbes

Growth of pathogens

  • Incubate at temperatures which do not encourage the growth of pathogens (not around 37oC).
  • Do not seal cultures completely before incubation (otherwise growth of anaerobes is encouraged), but make sure they cannot be opened accidentally.
  • Use sterile equipment and procedures.
  • Avoid draughts (from open windows and doors) which could contaminate cultures and cause fungal spores to spread.
  • Work near the updraft from a Bunsen burner flame to help prevent contamination of the cultures.
  • After work is complete, treat surfaces using a suitable disinfectant, for a suitable length of time.
  • Dispose of all cultures by sterilisation in an autoclave (pressure cooker).
  • Always wash hands after handling cultures and before eating food.

Molten MRS agar

Risk of scald

Handle with care