My family successfully grew bacteria in nutrient agar.
Our samples came from our hands after petting the dogs, swabbed samples from the fridge door, and an exterior door; then a sample of all the same surfaces after washing hands or wet-wiping handles.
The kids had this question:
Why do we see a growth on our homemade petri dish sample, but do not see growths on the surfaces from which the samples were taken?
The answer is that while bacteria may be found on different surfaces, they multiply only if provided necessary survival nutrients. Bacteria need water and food for energy, such as sugars, starch, protein and fats. Some bacteria need oxygen while others can live without oxygen. The surfaces you tested for bacteria did not have all the necessary requirements for bacteria growth, but the material in your petri dishes did.
CAUTION: Treat all bacteria cultures as pathogens (dangerous-harmful). Use a 10% Bleach solution to kill bacteria. How to make a 10% solution of bleach
1. A 10 % solution means that 10 parts solute is added to 90 parts of solute. The combined mixture totals 100 parts.
2. The ratio of 10/100 can be reduced to 1/9. Using the metric volume measurement of milliliters, mL, the mixture would contain:
1 mL of bleach + 9 mL of water producing 10 mL of a 10% bleach solution
FYI: Using cups add 1/4 cup with 2 1/2 cups of water= 23/4 cups of solution.