Aoife M. answered • 11/04/24
B.S. and M.S. Degree in Biology with College-Level Teaching Experience
Typically, when petri plates are created, they have a few important ingredients and nutrients needed for bacterial growth. In this explanation, I will be telling you about Tryptic Soy Agar (TSA) and using this as an example, since laboratory TSA is the most similar to the recipe you provided. If there are issues with any of these ingredients, it might be the cause for your liquefied plates.
In my experience of making thousands of these, plates like yours usually have issues with the mixing, heating/cooling process, or the amount of gelatin/agar.
Some ingredients commonly found in TSA agar are:
- Casein, sugar, beef extract, and/or soybean meal digest - these provide a nitrogen source, amino acids, vitamins, and nutrients for bacteria to grow.
- Table salt (NaCl) and phosphate - salt maintains the osmotic equilibrium for bacterial cells; phosphate keeps the pH suitable for microbial growth.
- Water, agar - this is used to create the jelly-like and solid petri dish growth medium. In TSA plates, it is usually derived from seaweed. The water is the solvent for dissolving the other ingredients.
It sounds like the ingredients in your petri dishes provide great amounts of nutrients for bacterial growth; I believe the sugar and beef granules in your recipe are not the issue here. It seems like your issue may be in your water/agar (aka gelatin) ratio; it is also likely that there is an issue with the temperature of your ingredients throughout the process. I have a few experience-based suggestions to make sure this doesn't happen again.
- TSA has pre-set and very exact amounts of each ingredient. The recipe may have had too much water or needed more gelatin powder.
- TSA is very finicky about temperature, mixing, and cooling process. It sounds simple, but I have messed up many plates before during this seemingly easy process! Your experience has happened to me when I did not mix the ingredients well enough (there were some gelatin bits, but the rest became liquid when I touched it with my loop). Make sure you heat it to boiling, stir until the liquid is clear, and you can't see any granulated ingredients. In addition, burning the liquid/over-boiling it will cause plates like this. If some of the gelatin gets burned during mixing, it doesn't dissolve and precipitates out, making your plates less solid. You really have to keep an eye on it while it is boiling, and take the pot off the stove ASAP once everything is dissolved.
- If the plates were liquid at one time and solid at another, they could have been heated or cooled too rapidly during the time you made them. Petri dishes should be left to cool at room temperature, as condensation from the hot plates can increase during the rapid temperature change from stovetop to fridge. The condensation will add extra water to your petri dishes, causing that texture change. (Side note: I usually stack my petri dishes on top of each other or near each other. This ensures that they share heat equally and none of them cool down too quickly, which causes additional condensation.)
In conclusion, I think your issue was likely caused by #2 or #3 - mixing or rapid temperature changes. To prevent this from happening again, make sure you don't accidentally burn any ingredients, watch it very closely while it is boiling and take it off the stove asap when done, and let the petri dishes cool at room temperature. You can also add probably a half packet more of the gelatin, but I would try this last.
Hope this helps you and others in your homemade microbial adventures :)
