Scientific basis for the project

The “five-second rule” states that food that is dropped on the floor is safe to eat if picked up within five seconds. Students are asked to design experiments to test whether this rule is true. They are given a list of consumables, prepared media, and bacterial cultures that they can work with, but otherwise are free to design their experiment however they wish.

Students are encouraged to try lots of different sampling methods. In our experience, the most successful involves transferring the bacteria from the contaminated object by putting it into a few millilitres of liquid, vortexing for a few seconds, and then plating out different concentrations onto LB agar plates. Colonies are counted following overnight incubation at 30°C (at this point the plates can be stored for 1-2 weeks at 4°C if there is a gap between sessions).

Students are often keen to sample real surfaces (such as the floor), but we have found that these can be a lot more variable than artificially contaminated surfaces. Furthermore, the natural flora of some real foodstuffs can make it difficult to determine what contamination the food has picked up off the floor, and so we now provide students with Serratia marcescens with which to inoculate an artificial surface (for example, a tile). S. marcescens grows as a bright red colony on LB agar and so is easy to distinguish from background flora.

Brief summary of each session

• Lab session 1. Test different sampling methods.
• Lab session 2. Apply the most successful sampling method to test several variables (type of foodstuff, type of surface, surface bacterial load, contact time).
• Lab session 3. Formulate a hypothesis and use the most appropriate sampling method and variable(s) to test the hypothesis. Set up a comprehensive experiment with suitable controls and technical/biological replicates.
• Lab session 4. Score and analyse the data. Discuss data with staff in preparation for writing a lab report.

Learning objectives

• To formulate a scientific hypothesis.
• To work as a team to design and perform an experiment to test the hypothesis.
• To use controls appropriately.
• To define the variables of an experiment.
• To interpret data appropriately.
• To present data in a clear and coherent way.
• To write up the data in a lab report.

If you decide to use this practical, please let us know about it by filling in this short form – it’s not a requirement but we’d love to hear how our ideas are being used!