6.14—Core Practical 14 - antibiotics and bacteria

To compare antibiotics, spread a known bacterial culture on sterile agar, place equal-sized antibiotic discs and measure the clear zones after the same incubation. A larger zone suggests greater inhibition under those conditions.

Use aseptic technique, a distilled-water negative control, equal disc size and concentration where possible, replicate plates and consistent incubation. Invert plates to reduce condensation and use safe temperature/containment.

If tetracycline produces a 14 mm zone and methicillin 8 mm on the same strain, tetracycline inhibited growth more in that assay; the result does not automatically establish clinical choice.

Zone diameter depends on diffusion, concentration, agar and bacterial growth rate, not only antibiotic potency. Do not compare zones from different protocols without calibration.