Scientists investigated the effectiveness of three different antibiotics (labeled S, P, and E) against a clinical strain of Staphylococcus aureus bacteria.
They placed paper discs containing these antibiotics onto a nutrient agar plate inoculated with the bacteria. They also placed a control paper disc, labeled C, containing only sterile distilled water.
The plate was incubated and the results are shown in the diagram below:
Explain the purpose of using the control paper disc C in this investigation.
The diameter of the clear zone of inhibition around antibiotic disc S is 34 mm34\text{ mm}34 mm.
Calculate the area of this zone of inhibition.
Use the equation: Area=πr2\text{Area} = \pi r^2Area=πr2
Take π≈3.142\pi \approx 3.142π≈3.142. Give your answer to 3 significant figures.
A few years later, clinical reports showed that this strain of Staphylococcus aureus was no longer affected by antibiotic S. Suggest why antibiotic S no longer had an effect.