Cultivate your own germs to determine what alcohol or soap does

 In the next activity, we want to learn about soap and hydroalcoholic gel effects on microorganisms. Which one is the most efficient to kill germs?

What are the consequences on microorganisms  when using soap or hydroalcoholic gel ?

Materials :

  • 3 premade agar plates for yeast growth
  • permanent marker pen
  • bunsen burner
  • soap and hydroalcoholic gel solutions
  • yeasts (Saccharomyces cerevisae)solution (104cells/mL)
  • water
  • rake (=râteau)
  • micropipette and tips

Procedure :

We want to grow yeasts with soap (S2), hydroalcoholic gel (A2) or nothing (T2) to understand these products effects on microorganisms growth.

  • -Switch/turn on the bunsen burner.
  • – Label three plates : yeasts with soap (S2), hydroalcoholic gel (A2) or nothing (T2) on the underside of each plate, using a marker pen. Add your names. When turning a plate upside down, keep the lid pressed down to avoid it falling off.
  • -Place the agar plates on the table (the correct way up) and remove the lid of the plate marked ‘T2’.
  • -Mix the yeasts solution and withdraw 25 μL using the micropipette.
  • -Deposit the drop in the middle of the plate and throw the tip.
  • – Withdraw 25 µL water (plate T2), soap (plate S2) or hydroalcoholic gel (A2).
  • -Deposit the drop in the middle of the plate and throw the tip.
  • -Replace the lid and clean your hands using soap. Dry them well using paper towels.
  • -Spread the 2 solutions (yeast+water, yeast+soap or yeast+hydroalcoholic gel) with the rake on the whole plate.
  • -Replace the lid.
  • -Leave plates in the incubator (28ºC) for 48–72 hours, or until you can see some bacterial colonies.

Observation and discussion :

  • Compare your ‘before’ (T1) and ‘after’ (S1 and A1) handprints.
  • Compare your yeasts culture with soap (S2), hydroalcoholic gel (A2) or nothing (T2).
  • What do you observe on your agar plates ? What are the dots?
  • Are there different types of dots (different colours and shapes)?
  • Is there a difference in microorganisms growth between the 3 plates? What does this show?
  • Count the dots on each plate to estimate the number of bacterial colonies. Record this in a classroom table and compare your results. 
  • Why do some plates have more growth than others?
  • What does this tell you about your hand hygiene, or the hygiene of your peers? 

Yeast cell structure :

Video : How to grow yeast ?

  • Circle the correct answer :
  1. soap /  hydroalcoholic gel kills yeasts.
  2. soap /  hydroalcoholic gel detachs yeasts.
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