5.18: effects of antiseptics core practical

What is the aim of the experiment?

Investigate the effect if antiseptics, antibiotics or plant extracts on microbial cultures

What is the method of preparing the agar plates of a colony of bacteria?

Glass Petri dishes and agar gel must be sterilised in an autoclave before use or pre-sterilised plastic Petri dishes can be bought. Use a sterile pipette to add a few drops of the microorganism solution to the agar, close the lid of the agar plate and place the pipette in disinfectant. Unwrap a sterile spreader (or sterilise it in ethanol) and use to spread the microorganism solution across the entire surface of the agar plate. Label and invert the plate and store upside down. Incubate at 25 degrees C.

Why should glass Petri dishes and agar gel be sterilsed?

This kills any bacteria present in the solution or on the Petri dishes

Why are agar plates used?

This provides the bacterium with the nutrients needed to grow

Why is microorganism solution added to the agar?

The solution contains the bacteria that will grow on the plate

Why is the microorganism solution spread across the agar plate?

It allows a lawn of bacteria to be produced across the whole of the plate

Why is the plate stored upside down?

This stops additional unwanted bacteria from the air from contaminating the plate. Do not fully seal the lid, as this will stop oxygen from reaching the bacterium and may encourage harmful anaerobic bacteria to grow

Why is the Petri dish incubated at 25 degrees?

It reduces the chance of growing harmful pathogens - although hospital laboratories would incubate the plates at 37 degrees (body temperature) to allow quick growth and identification

What is the purpose of adding antibiotic or antiseptic soaked patches to pre-prepared agar plates?

By adding filter paper soaked in a variety of anti-microbial solutions to the agar plate, the effectiveness of the solutions can be tested experimentally. A clear area (zone of inhibition) indicates that the bacteria have been killed.

What is the method for adding antibiotic or antiseptic soaked patches to pre-prepared agar plates?

Soak filter paper disks in a variety of anti-microbial solutions (using either different concentrations of the same solution or the same concentration of different solutions). Then measure the clear area around the soaked filter paper disk. Also prepare a control disk (with no solution) to compare with

Why are different concentrations of the same solution or a variety of different solutions of the same concentration used?

The effectiveness of each solution/concentration at killing the bacteria can be tested

Why is the clear area around the filter paper measured?

The size of the zone of inhibition indicates the effect of the substance tested on the growth of the specific bacterium