The first antibiotic, penicillin, was discovered by Alexander Fleming in 1928.
Since then over 100 antibiotic compounds have been discovered and used to save and extend
countless lives.
However, inappropriate use coupled with a shortage of newly developed antibiotics means
that bacteria have adapted and evolved to find ways to survive the effects of a many
essential antibiotics.
Antibiotic resistance is currently considered to be one of the greatest threats to patients’
safety in the developed world.
Here at the University of Birmingham a number of scientists are conducting research into
how antibiotic resistance develops in bacteria in an effort to tackle
This looming global crisis.
But, could any of the practical experiments that we get shown in our school classrooms
play a role in this fight against resistance?
One experiment we might be familiar with in the classroom is assessing the effect of different
plant extracts on the rate of bacterial growth on an agar plate like this.
Well, this same theory could and still is used by World leading researchers to test
how effective certain antibiotics are against different strains of harmful bacteria.
To test how effective our antibiotic is we’re going to use a Disk Assay where we soak small
discs of filter paper in different concentrations of antibiotics and then see what effect this
has on bacterial growth on an agar plate that’s been seeded with a thin layer of E.coli.
To prevent unwanted bacteria or other microorganisms from colonising our plate we need to adopt
an aseptic technique.
For this, my lab bench is spotlessly clean and I have a Bunsen burner on to ensure we
are drawing air currents upwards and away from my sterile petri dish.
We’re going to be testing the susceptibility of our bacteria to different concentrations
of the antibiotic ampicillin.
I’ve made serial dilutions to give ampicillin solutions of 1.5mg/ml and 0.15mg/ml.
To make a 0.15mg/ml solution from the 1.5 mg/ml solution, you need to dilute 1 volume
of the 1.5mg/mL Ampicillin with nine volumes of water.
I’m then going to add a small volume of each solution to a small, sterile disk of
filter paper.
I need to ensure I add the same volume of each solution to reduce variability in the
experiment.
I’m also including a control disc to which I’m just going to add water.
Once the antibiotic has been absorbed and dried on the filter paper, I can add each
of disks to the agar plate.
Once the disks have been added, we can incubate our plate overnight at 37 degrees C and see
if we have an effect on bacterial growth.
Here we have our agar plate that’s been incubated overnight.
We can clearly see that around the antibiotic disks we have a clear zone of inhibition where
bacterial growth has been inhibited by the antibiotic.
The antibiotics at a greater concentration also have a larger zone of inhibition than
those at a lower concentration and no zone of inhibition is seen around our negative
control.
If I compare this plate to a plate with a strain of bacteria that is resistant to ampicillin,
we can see that there’s no zone of inhibition around any of the antibiotic disks.
With the increasing prevalence of antibiotic resistant strains of bacteria such as this
it’s imperative that researchers find out how these bacteria develop their resistance
to help us to develop new ways to treat bacterial infections and stay one step ahead of resistance.