In this two-part lab, students will sample the indoor environment to estimate how many bacteria are living on surfaces, then test the effectiveness of various household disinfectants on bacterial growth. Students use RODAC petri plates to directly sample surfaces such as benches, desks, or cell phones and then can quantify the number of bacteria detected before and after cleaning with a disinfectant. They will also set up a disc diffusion assay to test the potency of household cleaners and other disinfectants against Gram-positive and Gram-negative bacteria.

Putting Disinfectants to the Test (PDF)


Intended Audience








Learning Objectives

At the completion of this activity, students will be able to:

  1. Quantify the number of cultivable bacteria from an environmental surface sample
  2. Observe how well different chemical disinfectants can kill bacteria or inhibit bacterial growth
  3. Compare the effectiveness of disinfectants on Gram-positive vs. Gram-negative bacteria
  4. Discuss the importance of disinfecting surfaces at home, especially in the kitchen


Student Background

Students should be familiar with lab safety and how to measure diameter. Students should also know that bacterial colonies originate from a single bacterial cell that grows and divides.



Antimicrobial, Environmental sampling, Disc diffusion, Colony forming unity (CFU)


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