Intro: Cook Your Own Microbial Growth Plates
The goal of this hands-on project was to make and validate microbial growth plates with ingredients and tools commonly found in a kitchen.
I performed this with my first year bachelor students in dietetics and nutrition.
By making these plates the students get a better understanding of what microbial organisms need as an energy and nutrient source.
Also will they have to perform standard methodes and practices (working sterile, waste managment,...) applied to microbiology labs in order to achieve the goal.
By interpreting the results students will learn to analyze and be critical. They will also learn a bit more about differential (growth) behaviour of microbes.
By using common kitchen ingredients and tools, microbiology is taken out of the lab and placed into the dietician's world. I hoped by doing this microbiology would become less abstract and more fun to them.
I tried this project with bachelor students in dietetics and nutrition, but I guess it is suited for other students as well.
This is a low-cost project that can be performed in any basic kitchen.
My students were enthousiastic, it was fun to do!
Step 1: Ingredients
We decided to make a microbial growth plate based on the growth plate YPD routinely used to grow yeast in research labs.
YPD consists of 1% yeast extract, 2% peptone and 2% glucose.
Therefore we used 1% dry yeast, 3% milk and 2% sugar in our growth medium.
As solidifying agent we used gelatin (10%) instead of agar.
We also added salt (0,9%) since we want to grow bacteria on our growth medium as well.
Step 2: Mixing the Ingredients
- Weigh precisely, for a final volume of 150ml:
salt: 1,35 grams
sugar: 3,0 grams
dry yeast: 1,5 grams
milk: 4,5 milliliters (mls)
- Put these 4 ingredients in a beaker (or alternatively a pan) and add 100 mls of water.
- Heat the mixture (we used a bunsenburner, but alternatives are a kitchenburner or a microwave oven) to better homogenize everything.
- Weigh 15 grams of gelatin leaves and soak 1 minute in cold water.
- Take the soaked gelatin leaves out of the water
- Make sure to thoroughly wringe the gelatin leaves
- Add the gelatin leaves to the mixture
You will notice that by adding the gelatin to the mixture, its volume will increase with 50%, from 100 mls to 150 mls.
Step 3: Pour the Mixture in Jars
We used cleaned jam jars to which we added about 40 mls of our growth medium mixture.
Step 4: Sterlize the Growth Medium
For sterilization we used an old high pressure pan.
- First, pour 500 mls water, preferably destilled, into the pan
- Next, place the closed jars with medium into the pan. CAUTION: do NOT close the lid of the jar tightly! Otherwise the jars may explode while cooking!
- Additionally: put a piece of sterilization indicator tape on the lid.
- Steam the jars for 20-25 minutes into the high pressure pan.
Step 5: Let the Growth Medium Solidify
After sterilization, the growth medium is liquid. In order to become solid it has to be put into the fridge.
This will take at least 30 minutes.
To increase the solidification process, the jars can be placed into an ice bath.
Make sure not to open the lid, the medium is sterile now.
Step 6: Inoculate the Solidified Growth Medium
The growth medium is sterile and since we want to avoid contamination, the inoculation should be done in the vicinity of a bunsenburner. Alternatives for a bunsenburner are a camping gas burner or a kitchen torch. If none of these are available, just rapidly inoculate and rapidly close the lid afterwards.
We inoculated the growth media with different microbial sources:
- One jar was inoculated with baker's yeast, E. coli and Staphylococcus aureus.
*This was done by plunging a sterile (ear) swab into a tube with either baker's yeast, E. coli or Staphylococcus aureus and next
to "draw" a straight line with the soaked swab on the growth medium.
*(Ear) swabs can be sterlized in an closed jar using a high pressure pan as well.
- Another jar was inoculated with the residual yeast found in (Belgian) beers.
The bottle was slowly decanted, the residual yeast was poured into a beaker and part was pipetted on the growth medium
- An additional jar was inoculated with a swab that was used to sample the region behind one's ear. But you could also swab a nose,
a doorknob, ... or put a finger directly on the medium.
- Finally, one jar was not inoculated, this was our negative control to verify the sterilization process.
Step 7: Incubate the Jars
Incubation was done for 4 days at 25 degrees Celsius.
Make sure to place the jars for 4-5 days into a closet, or whatever, with a constant room temperature (20-25 degrees C).
The temperature may not exceed 28 degrees C!
Above 28 degrees C gelatine will become liquid again!
Step 8: Check Results
Our experiment was successful!
We clearly see that the bacteria E.coli and S. aureus grow well on our growth medium.
S. aureus is known to digest gelatin and this is also noted: the gelatin becomes liquid again at the place where a line was drawn with a swab containing S.aureus.
Yeast however grows poorly. We were also unsuccessful in our attempt to isolate yeast from beer.
Next school year I will organise another attempt to successfuly grow yeast with my students, I'll let them try other nitrogen sources than milk, plus we will cut out the salt.
The medium was inoculated with the swab sampling the ear.
Little colonies are seen to grow on the medium!
Additionaly we notify the unpleasant odour of the lactose in the milk being fermented to lactate
No growth on the negative control!