Antiseptics/antibiotics required practical

"Investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring zones of inhibition."

Antibiotics are chemicals produced by microorganisms that can kill bacteria. Different antibiotics work against different bacteria, so it's important to test their effectiveness. When an antibiotic is placed on agar jelly, it diffuses outwards and can prevent bacterial growth or kill existing bacteria nearby.

Steps to Compare Different Antibiotics

1. Prepare the Petri Dish:

   • Mark the underside of a Petri dish with six spots in a circular pattern and label them A to F.

2. Spread Bacteria on the Agar:

   • Pour nutrient agar into the Petri dish using aseptic techniques.

   • Use a sterilised loop to spread a bacterial species evenly across the agar surface.

3. Apply the Antibiotics:

   • Soak small filter paper discs (1 cm diameter) in six different antibiotics, each at the same concentration.

   • Place each disc on an agar spot (A-F) and note which antibiotic is on each spot.

4. Incubate:

   • Incubate the dish at 25°C for two days.

5. Measure and Calculate:

   • Measure the diameter of the clear area around each disc where bacteria haven’t grown (this is called the inhibition zone).

   • Calculate the area of each inhibition zone using the formula: πr², where π is approximately 3.14, and r is the radius (half the diameter).

Additional practical skills

• If the radius of the inhibition zone is 8 mm, the area can be calculated as:

• Area = 8 x 8 x 3.14 = 201mm² 

• The larger the inhibition zone, the more effective the antibiotic is at killing or preventing the growth of that specific bacterial species.