Bacteria Science Fair Project - How Much Bleach to Kill Bacteria?

If you’ve ever gone camping and had to wash your dishes without hot water, you know that washing them in diluted bleach will kill the germs. But because bleach can also have some negative effects, Jaleesa Chavez wanted to find out the least amount of bleach that was needed to still kill bacteria.

She summarized the project in her display, shown in the figure.

Figure: Project display for “What is the weakest solution of bleach that will kill bacteria?”

Hypothesis

I believe that I can find the smallest strength of bleach needed to kill E. coli bacteria.

Experimental variables

Strength of bleach solution applied to E. coli bacteria

Measured variables

Percent of E. coli killed by bleach solution

Controls

Amount of solution applied to E. coli bacteria

Experimental groups

• 3 percent bleach solution
• 5 percent bleach solution
• 8 percent bleach solution
• 10 percent bleach solution
• 15 percent bleach solution

Control groups

Water

Materials

• Bleach
• Water
• 3 percent bleach solution
• 5 percent bleach solution
• 8 percent bleach solution
• 10 percent bleach solution
• 15 percent bleach solution
• E. coli bacteria (Purchase from Carolina Biological Supply or Presque Isle Cultures)
  • NOTE: In order to properly manipulate bacteria, you need aspetic techiques and the right tools. It is recommended that you work closely with your science teacher or reach out to a local communty college or university for guidance on working with E. coli.

Procedures

1. Create solutions of 3 percent, 5 percent, 8 percent, 10 percent, and 15 percent bleach in water.
2. Spray each solution on E. coli bacteria.
3. Measure amount and percentage of E. coli bacteria killed by the bleach solution.

Results

• The 3 percent bleach solution killed 1.75 percent of the bacteria.
• The 5 percent bleach solution killed 2.4 percent of the bacteria.
• The 8 percent bleach solution killed 2.6 percent of the bacteria.
• The 10 percent bleach solution killed 2.7 percent of the bacteria.
• The 15 percent bleach solution killed 4.1 percent of the bacteria.

Conclusions

My hypothesis stated that I could find and document the smallest concentration of bleach in water that would kill E. coli bacteria. Of the solutions tested, the 3 percent solution was the smallest concentration that will kill bacteria, although the amount of germs killed went up in proportion to the concentration of bleach.

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