Antiseptic

If a filter-paper disc containing an antiseptic or antibiotic is placed on the agar, the substance diffuses outwards. Where enough of it prevents bacterial growth, a clear zone of inhibition appears around the disc. Comparing these zones lets you investigate how different treatments affect the bacteria on the plate.

Bacteria can grow across nutrient agar in a Petri dish, forming a visible covering. This practical tests whether an antiseptic, antibiotic or plant extract reduces that growth. An antiseptic is used on living surfaces such as skin; an antibiotic is used to treat bacterial infections; and plants produce chemicals that may defend them against pests and pathogens.

Antibiotics and antivirals are used to treat particular kinds of infection, while antiseptics help prevent microorganisms entering tissues through places such as cuts.