Biology

2.9 - Culturing Microorganisms

2 min

note 1 of 1

Calculating the area of a colony or clear zone

A bacterial colony and a clear zone are both roughly circular, so their size is measured as the area of a circle.
A clear zone is the region around an antimicrobial-soaked disc where the substance has stopped the bacteria growing, leaving the agar clear.
The larger the clear zone, the more effective the substance has been against the bacteria.
Measuring the area turns a rough by-eye comparison into a numerical one, so different colonies or treatments can be compared accurately.

A round petri dish seen face on, its cream surface labelled bacterial colony, with two small purple discs labelled antimicrobial disc, each ringed by a yellow halo labelled clear zone.

Finding the area

Formula for area of a circle:

$Area = π r^{2}$

Where:

$r$ = Radius, the distance from the centre to the edge, in millimetres (mm) or centimetres (cm)
$π$ = A constant, about 3.14

Find $r$ by measuring the diameter across the colony or clear zone and halving it. Measure the diameter through the bottom of the closed Petri dish, without opening it, so the culture is not disturbed. The area comes out in square units, such as square millimetres (mm²) or square centimetres (cm²).

Worked example - Comparing two clear zones

Two antimicrobial discs are tested. Treatment A produces a clear zone 14 mm across; treatment B produces a clear zone 20 mm across. Calculate each area and state which treatment is more effective.

Step 1 - Find each radius

Treatment A: radius = 14 ÷ 2 = 7 mm

Treatment B: radius = 20 ÷ 2 = 10 mm

Step 2 - Apply the formula

Treatment A:

$Area = π \times 7^{2} \approx 154 {mm}^{2}$

Treatment B:

$Area = π \times 10^{2} \approx 314 {mm}^{2}$

Step 3 - Interpretation

Treatment B's clear zone (about 314 mm²) is roughly twice the area of treatment A's (about 154 mm²), so treatment B is the more effective of the two.

key terms

TermDefinition

Clear zoneThe region around an antimicrobial-soaked disc where the substance has stopped the bacteria growing. It is also called the zone of inhibition.

end of notes

Key points to remember

Calculating the area of a colony or clear zone

A bacterial colony and a clear zone are both roughly circular, so their size is measured as the area of a circle.

A clear zone is the region around an antimicrobial-soaked disc where the substance has stopped the bacteria growing, leaving the agar clear.

The larger the clear zone, the more effective the substance has been against the bacteria.

Measuring the area turns a rough by-eye comparison into a numerical one, so different colonies or treatments can be compared accurately.