Introduce bacteria to the Petri dishes. Once the agar solution has hardened and the Petri dishes are at room temperature, you are ready for the fun part – introducing the bacteria. There are a couple of methods of doing this – through direct contact or through sample collection.
Direct contact: This is when bacteria are transferred to the Petri dish using direct contact, i.e. touching the agar. One of the most common ways of doing this is to simply press your fingertip (either before or after washing your hands) lightly onto the surface of the agar. However, you could also try pressing a fingernail or the surface of an old coin into the agar or even placing a small hair or drop of milk into the dish. Use a sterile cotton swab if you have one available. Use your imagination!
Sample collection: With this method, you can collect bacteria from almost any surface and transfer it to the Petri dish, all you need are some clean cotton swabs. Simply grab a swab and swipe it over of any surface you can think of – the inside of your mouth, a door handle, the keys on your computer keyboard, or the buttons of your remote control – then use it to streak the surface of the agar (without tearing it). These places harbor a lot of bacteria and should provide some interesting (and disgusting) results in a couple of days time.
If you like, you can place more than one sample of bacteria in each Petri dish – all you need to do is divide the dish into quadrants (quarters) and swipe a different sample of bacteria in each of them.
It is recommended that you keep one quadrant of the Petri dish free of bacteria samples to use as a control group. This allows you to know if the agar had been contaminated prior to the introduction of bacterial samples.
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