INSTRUCTOR: Hello, and welcome to this core practical video. My name is Mr. Osborne, a
science teacher at Claremont High School in London. Today, I'm going to take you through
one of the core practical required for GCSE biology. The title of this practical is the
effect of antibiotics on microbial cultures.
Your teacher will advise you on the type of antibiotics, antiseptics, or plant extract
that you will be using for your experiment. To carry out this experiment, you will need
the following equipment. Petri dish with lid, screw top bottle of sterile liquid nutrition
agar, keep in water bath until needed. Bacterial culture in screw top bottle, sterile pipette,
sterile spreader in rubber, beaker of disinfectant, two small filter paper discs of different
antibiotic concentration, small disc that's sterile, filter paper, sticky tape, marker
pin, forceps, ruler, ethanol, Bunsen burner, and heat resistant mat.
I am now going to demonstrate how to successfully pour an agar plate. Keeping the lid on the
Petri dish, turn the dish upside down. Use the pen to draw three sections on the base.
Label one section control and the other two sections with the name or concentration of
each antibiotic or antiseptic solution.
Add your initials and the date near the edge of the dish. Turn the dish the right way up.
Safely light your Bunsen burner, making sure to keep the valve closed.
Remove the liquid nutrition agar from the water bath and dry using a paper towel. Unscrew
the cap of the nutrient agar bottle and quickly pass the glass neck of the bottle through
the Bunsen flame. Then lift the lid of the Petri dish just enough for the agar to be
pulled carefully into the dish. Poor enough agar to half fill the depth of the Petri dish
base, replace the dish lid immediately.
Leave the agar to solidify. You can also use a pre-prepared agar plate to save some time
or pour the agar plate in pairs. I am now going to demonstrate how to successfully make
bacteria plate. Unscrew the cap of the bottle of bacteria culture and quickly pass the neck
of the bottle through the Bunsen flame. Draw a small amount of control into the pipette,
then flame the neck of the bottle again and replace the lid.
Lifting the lid of the Petri dish as little as possible, gently add a few drops of culture
to the agar. Replace the dish lid and place the pipette in a disinfectant. Unwrap the
spreader or disinfect, and lifting the dish lid as little as possible, gently spread the
culture drops across the ego using a back and forward motion with the spreader. Replace
the dish lid and disinfecting the spreader.
I am now going to demonstrate how to successfully add antibiotic disks. Sterilize the forceps
by dipping them into ethanol then passing them through the Bunsen flame to ignite the
ethanol. Use the forceps to pick up the sterile filter paper disk. Lift the dish lid just
enough so that the forceps do not touch the bacteria layer on the agar in the dish. And
add carefully the disk on the section labeled control.
Replaced the dish lid as quickly as possible. Sterilize the forceps again. You are now ready
to prepare your antibiotic disks. Use your scissors to remove the specific antibiotics
you wish to investigate.
Sterilize the forceps by dipping them into ethanol then passing them through the Bunsen
flame to ignite the ethanol. Then repeat the previous step with the two of the antibiotic
disk, placing it on the appropriate section of agar. Remember to replace the dish lid
as quickly as possible.
Sterilize the forceps by dipping them into ethanol then passing them through the Bunsen
flame to ignite the ethanol. Tape the lid onto your Petri dish with two pieces of tape
and invert the dish. Leave it at 20 to 25 Celsius for two to three days. To record your
results, you need to look carefully at your dish. Do not open it.
Measure the diameter of the circle around each disc, where there is no bacterial growth.
Calculate the zone of inhibition using an appropriate formula. Use your data on your
own knowledge of antiseptics, antibiotics, and plant extracts to explain the results
you have obtained during your experiment.