Core practical - effect of antiseptics, antibiotics or plant extracts on microbial cultures The effectiveness of antibiotics Flashcards
Method A - Preparing the agar plates of a colony of bacteria (6)
1- Glass Petri dishes and agar gel must be sterilised in an autoclave before use or pre-sterilised plastic Petri dishes can be bought.
2- Pour the sterile agar plates and allow to fully set.
3- 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.
4- Unwrap a sterile spreader or sterilise a spreader in ethanol. Use the spreader to spread the microorganism solution, across the entire surface of the agar plate.
5- Label and invert the plate, and store upside down.
6- Incubate at a maximum temperature of 25°C in schools and colleges.
why must Glass Petri dishes and agar gel be sterilised?
this will kill any bacteria that are present in the solution or on the Petri dishes.
why do you need to Pour the sterile agar plates and allow to fully set.?
this provides the selected bacterium with all the nutrients needed to grow.
why do you need to ‘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.’?
this solution contains the bacteria that will grow on the agar.
why do you need to ‘Unwrap a sterile spreader or sterilise a spreader in ethanol. Use the spreader to spread the microorganism solution, across the entire surface of the agar plate.’?
this allows a lawn of bacteria to be produced across the whole of the plate. Replace the lid as soon as possible, secure with tape.
why do you need to ‘Label and invert the plate, and store upside down.’? (3)
-this stops additional unwanted bacteria in the air contaminating the plate.
-Do not fully seal the lid, as this will stop oxygen reaching the bacterium, and this may encourage harmful anaerobic bacteria to grow.
-Labels are important, as this identifies the growing bacterium.
why do you need to ‘Incubate at a maximum temperature of 25°C in schools and colleges’? (2)
-this reduces the chance of growing harmful pathogens.
-Hospital laboratories would incubate plates at 37°C (body temperature) to allow quick growth and identification.
what indicates that the bacteria have been killed?
A clear area (zone of inhibition)
Method B - Adding antibiotic or antiseptic soaked patches to pre-prepared agar plates + REASONS (4)
1- Soak filter paper disks in a variety of solutions, use either different concentrations of the same solution, or a variety of different solutions.
REASON: the effectiveness of the solutions at killing the bacteria can be tested.
2- Measure the clear area around the soaked filter paper disks. A control disk must be also included.
REASON: size of zone indicates the effect of the substance tested on the growth of the specific bacterium.
how could we calculate the effect of antibiotics or antiseptics on bacterial growth?
use ‘pie r squared’ to calculate the area of the clear zone
