A bacteria growth and hygiene project lets students compare how many microbes are found on everyday surfaces such as phones, sinks, door handles, and hands. By swabbing surfaces onto agar plates, students can see visible colonies form over several days. The project matters because it connects invisible microorganisms to real hygiene choices, cleaning methods, and public health.
It also teaches experimental design, careful observation, and safe lab habits.
Key Facts
- Each visible colony usually begins from one bacterial cell or a small group of cells called a colony-forming unit.
- Colony density can be compared using CFU per area: CFU/cm^2 = number of colonies / swabbed area in cm^2.
- A controlled experiment changes one main variable at a time, such as surface type or before versus after cleaning.
- Percent reduction after cleaning can be calculated as percent reduction = ((before count - after count) / before count) x 100.
- Agar provides water and nutrients so bacteria can grow into visible colonies under suitable conditions.
- Sealed plates should not be opened after incubation because unknown microbes may be harmful.
Vocabulary
- Agar
- A gel-like material used in petri dishes to provide a surface and nutrients for growing microorganisms.
- Bacterial colony
- A visible cluster of bacteria growing on agar that usually came from one cell or a small group of cells.
- Variable
- A factor in an experiment that can be changed, measured, or controlled.
- Control group
- A comparison setup kept under standard conditions so results from the test groups can be judged fairly.
- Incubation
- The period when plates are kept under conditions that allow bacteria to grow.
Common Mistakes to Avoid
- Opening plates after colonies grow is unsafe because unknown bacteria or fungi may be released into the air or onto skin.
- Changing surface type and cleaning method at the same time makes the results hard to interpret because you cannot tell which variable caused the difference.
- Counting only the largest colonies gives biased data because smaller colonies also represent microbial growth.
- Comparing plates photographed at different times is misleading because colonies grow over time and older plates usually show higher counts.
Practice Questions
- 1 A phone swab plate has 84 colonies before cleaning and 21 colonies after cleaning. Calculate the percent reduction in colony count.
- 2 A student swabs a 25 cm^2 area of a sink and counts 150 colonies after incubation. What is the colony density in CFU/cm^2?
- 3 A class compares a door handle before cleaning, a door handle after cleaning, and an unused agar plate. Explain why the unused plate is important and what it shows if colonies grow on it.