Invisible germs become numbers you can compare once they grow into colonies on a plate. In this experiment, test subjects carry labeled water bottles for three days, refilling them each morning without washing. On day three, you filter water samples through Nalgene filtration units, place the filters in petri dishes with melted BHI agar, and incubate for 24 hours. An unwashed bottle can show colony counts thousands of times higher than the sealed control.

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.

Each colony on an incubated plate represents a viable cell that survived UV exposure, so the count tells you directly how effective the treatment was. You collect water samples from a local creek, expose them to a UV lamp, and pull samples at four time points over 24 hours. Each sample goes to a microbiology lab for incubation and counting. With a low-wattage lamp, bacteria decreased at first but then rose above the original count. A higher-intensity lamp produced a steady decline throughout the full 24 hours — consistent with the formula E = W / D-squared, which explains why both lamp power and distance matter.

Colony counting is the standard way to judge whether a disinfectant actually worked: you spread treated bacteria onto agar plates, incubate them overnight at 35 degrees Celsius, and count what survives. Here, four household products — including bleach and Lysol — go head to head against six bacterial species. You mix each disinfectant with bacteria in sterile tubes, wait five minutes, then spread the mixture onto nutrient agar plates. The next day, the plate with the fewest surviving colonies identifies the product that killed the most bacteria.

Counting bacterial colonies on an agar plate tells you how many germs were in a water sample. In this experiment, you swab water from three different sources onto agar dishes before and after leaving each bottle in direct sunlight for two days. After five days of growth, you measure the colonies. Smaller groups on the sun-exposed plates mean the water had fewer bacteria — showing how much ultraviolet rays can reduce bacterial growth.