A student wanted to investigate how effective three different antiseptics were at killing bacteria. The student was provided with an inoculated Petri dish prepared using the method in Figure 10 ,three different antiseptics , filter paper discs and sticky tape. Devise a plan for the student to complete this investigation. Include a control and any variables that the student would need to consider – 10031

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1. To find out which antiseptic works best, I would soak one paper disc in antiseptic 1, then do the same with new discs for antiseptic 2 and 3. I would also soak a disc in water as a control. I’d put the discs in different parts of a bacteria-covered agar plate, label it, and incubate it.
   After that, I’d measure the clear area around each disc (called the zone of inhibition). I could work out the area using πr². The biggest clear zone shows which antiseptic killed the most bacteria.
   To stay safe, I’d work near a Bunsen burner, lift the lid only a little, wash my hands, and tape the lid to the dish. I’d keep things like disc size, amount of antiseptic, and incubation time the same to make it a fair test.

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