Alexander Fleming’s chance discovery of the antibiotic effects of Penicillium mould led to one of the greatest medical advances. You can investigate this effect by using a petri dish of agar jelly on which a lawn of E. coli bacteria has been grown. The lawn of bacteria has a milky colour but if the bacteria are killed a clear space appears.

What to do

• Get a petri dish of agar jelly with bacteria growing on it.
• Use a waterproof marker to mark sections on the bottom of the dish. You need one section for each antibiotic you are testing and an extra one as a control.
• Label each section with the name of the antibiotic. Add your name and the date.
• Sterilise a pair of forceps in a flame and place an antibiotic disc in each section. Do not put anything in the control section.
• Seal the lid with tape.
• Incubate your plate for two days at 25^oC.

Reliable measurements
Think of a way of reliably measuring the effect of each antibiotic.

Questions
1. Which antibiotic was most effective?
2. What was the purpose of the control?
3. How could you use this method to compare the effect of household disinfectants?