A Salty Snack for E. coli: The Efficacy of the 72-hour Rule

Overview

Objectives/Goals The objective of our project was to try and test the efficiency and credibility of the 72-hour rule by growing E.coli in different concentrations of seawater and measuring there growth over time. For those unfamiliar with what the 72-hour rule is, it is a merely a suggestion for how long someone should stay out of the ocean after a rainstorm. Methods/Materials To conduct our experiment, an LB Broth solution was made with de-ionized water and sea water to provide nutrients to the bacteria. Two colonies were then added to the 5mL of broth and incubated for 3hrs. The five different concentrations were made using the LB Broth; the broth was split into culture tubes with 5mL each. These tubes were autoclaved. 100 micro-liters of the incubated E. coli was added to each of the culture tubes. The leftover broth was used to make blanks that would be used in the spectrophotometer. After 24hrs the solutions were tested for growth in a spectrophotometer. This process was repeated every 24hrs for 72hrs and again at 120hrs. Results Our results show that E.coli was still growing steadily in all concentrations of seawater. Conclusions/Discussion Due to the fact that there was still E.coli growth in the seawater, this proves that there is a potential that the 72-hour rule may need to be extended. However, in a real life situation, the E.coli in storm water runoff would be competing with other bacteria and pollutants which may inhibit its growth.

Competition history

• CSEF 2011 Microbiology (General) · Entry S1507

Resources

• CSEF project summary ↗

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