Serial dilution practice › Zone of inhibition

Zone of inhibition practice problems

Invented agar plates with antibiotic discs on a 5 mm grid: measure each clear zone, work out its area with πr² and check your answers, or print a key.

Level

Set

Invented plates and antibiotics, for practice only. Tap a plate to see it larger. Diameters within 1 mm count.

    How the plates are drawn

    Each plate is a 90 mm dish covered in a lawn of bacteria, with four 6 mm paper discs centred on grid lines. Clear zones are whole millimetres and at least 3 mm apart in size, so the best antibiotic is never a close call. At AP / A-level and college one disc is soaked in sterile water as a control, and the area asked for is the clear agar only, without the disc.

    A worked zone

    In set ZAREA2 (high school), the clear zone around disc C spans 9 mm, just under two grid squares. The radius is 4.5 mm, so the area is π × 4.5² = 63.6 mm². Disc B has the widest zone, 25 mm, so its antibiotic worked best; disc D has none.

    Open set ZAREA2

    Questions

    How do you measure a zone of inhibition?

    Measure the clear circle across its widest part, edge to edge through the middle of the disc, in millimetres. On a real plate, use a ruler under the dish. Here the plates sit on a 5 mm grid with a ruler below.

    How do you calculate the area of a zone of inhibition?

    Halve the diameter to get the radius, then use area = πr². A clear zone 18 mm across has r = 9 mm, so its area is 3.14 × 9 × 9 = 254 mm². For the clear agar only, take away the disc's own area too.

    What does a bigger zone of inhibition mean?

    On one plate, a wider clear zone means that disc stopped more of the bacteria growing near it. No clear zone at all means the bacteria grew right up to the disc.

    Why is there a disc soaked in water?

    It is the control. Same paper, no antibiotic. If no clear zone forms around it, the zones around the other discs came from what was on them, not from the paper.

    Is this the GCSE biology required practical?

    Yes. AQA GCSE Biology required practical 2 investigates antiseptics or antibiotics on agar plates by measuring zones of inhibition, and the specification asks students to calculate clear areas using πr². A-level required practical 6 uses aseptic technique for the same kind of test.

    Back to serial dilution and plate count practice.

    Sources, checked October 24, 2026: AQA GCSE Biology 8461 specification, version 1.0 (April 21, 2016, PDF); AQA AS and A-level Biology 7401/7402 specification, version 1.5 (November 26, 2021, PDF); FDA BAM chapter 3 (January 2026), Petri dishes at least 15 × 90 mm.