Agar plate culture provides a flat dish of jelly-like food that lets germs collected from real surfaces grow into visible colonies you can count. In this project, swabs from restroom door handles transfer any microbes onto blood agar plates, where the jelly food supports bacterial growth during a 48-hour incubation at 35 degrees Celsius. The resulting colonies on each plate become a measurable record of how much bacteria was living on that handle.

Each colony on a blood agar plate represents bacteria that came from a single touch. You swab inside and outside handles at six locations across homes, restaurants, and supermarkets, then incubate the plates at 35 degrees Celsius for 48 hours before counting. The inside handle carries about 50 percent more bacteria than the outside — a difference that only becomes visible once those invisible microbes grow into groups you can tally. A Clorox disinfectant wipe reduces the colony count to zero at every location.

Incubation means keeping something warm, moist, and aired so living things inside can grow. Bacteria need these same conditions. Swabs go onto blood agar plates and sit in an incubator at 35 degrees Celsius for 48 hours. That steady warmth and moisture let tiny bacterial colonies multiply until they are large enough to count by eye.