Required Practical: Growth Flashcards

All 6 cards in this deck

What is a zone of inhibition?

The clear area around a disc on an agar plate where the bacteria have not grown.

How do you calculate the cross-sectional area of a zone of inhibition?
e.g. a clear zone of diameter 8 mm

1. Halve the diameter to get the radius: $8 \div 2 = 4$ mm
2. Use $\pi r^2$: $\pi \times 4^2 = 50.3$ mm$^2$

What does a larger zone of inhibition around a disc show?

That the antiseptic or antibiotic is more effective at killing or inhibiting the bacteria.

Why is a disc soaked in sterile water included on the agar plate?

As a control, to show that any zone of inhibition is caused by the antiseptic or antibiotic and not by the paper disc.

True or false? To use $\pi r^2$ for a zone of inhibition you must halve the measured diameter first.

True. The formula uses the radius, which is half the diameter.

Name two variables that must be kept the same when comparing different antiseptics using zones of inhibition.

e.g. the same bacterial culture and volume on each disc, and the same incubation temperature and time.