Required practical: antiseptics/antibiotics on bacterial growth

What you need to know

• Spread bacteria evenly over agar to form a lawn, then place paper discs soaked in different antiseptics or antibiotics onto the surface.
• A clear zone of inhibition forms where bacterial growth has been inhibited; under the same controlled conditions, a larger zone indicates a more effective treatment against that bacterium.
• Use aseptic technique throughout: sterilised media and dishes, a flamed inoculating loop, minimal opening of the plate, and a lid secured with tape after inoculation.
• Measure inhibition zones and use area = πr² when a quantitative comparison of circular clear zones is needed.

The big picture

This required practical investigates how antiseptics or antibiotics affect bacterial growth on agar plates. Bacteria are spread evenly to form a lawn, treated discs are placed on the agar, and clear zones of inhibition are measured. A larger zone indicates greater inhibition when conditions are kept the same. Aseptic technique, suitable low-risk bacteria and incubation at a maximum of 25 °C reduce contamination and safety risks.

Know the practical language

Get these terms clear before following the method.

From culture to measurement

Follow the practical in order and notice where contamination and safety are controlled.

1. 1

   Start aseptically

   Sterilised media and dishes and a sterilised flamed inoculating loop reduce contamination of the culture.

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2. 2

   Build an even lawn

   Spreading the bacteria evenly across the agar creates an even layer of bacterial growth.

3. 3

   Add the treatments

   Place paper discs soaked in different antiseptics or antibiotics onto the bacterial lawn.

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4. 4

   Secure and incubate

   Secure the lid with tape rather than sealing it completely, then incubate at a maximum of 25 °C.

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5. 5

   Measure the clear zones

   Measure the zones of inhibition around treated discs to compare how strongly bacterial growth was inhibited.

Measure a clear zone

Which treatment inhibited more?

Assume both treatments were tested under the same controlled conditions.

Treatment A produces a clear zone with radius 6 mm and Treatment B produces one with radius 3 mm. Which treatment inhibited bacterial growth more?

Memory hook

LAWN → DISC → CLEAR ZONE → MEASURE: grow an even lawn, add treated discs, find clear zones, then measure them.

Check yourself

Why would comparing inhibition-zone size be unfair if the treatments were not tested under the same controlled conditions?