Bacterial lawn
Learning objectives
• To be able to describe how to measure
the concentration or potency of a
substance by its effect on living cells or
tissues (bioassay)
• To be able to describe the use of a
bacterial lawn as a bioassay technique to
determine the effectiveness of
antibiotics and disinfectants on inhibition
of bacterial growth.
• A bacterial lawn is a layer of bacteria
growing on the surface of an agar plate.
• This is used to test the effectiveness of
antimicrobial substances such as
antiseptics or antibiotics, using a disk
diffusion test There are two ways of
making a bacterial lawn.
Pour plate method
• In the pour plate method a universal
bottle of warm, sterile, molten agar is
inoculated with bacteria, stirred, and
then the agar is poured into a plate,
where it sets. The bacteria will grow
uniformly on the surface.
Spread plate method
• In the spread plate method an agar
plate is inoculated with a small volume of
broth culture, which is then spread even
over the surface using a glass spreader
or a cotton bud.
Advantages and disadvantages
• The pour plate method
– Advantage:
• gives a more even lawn,
– Disadvantage:
• requires universal bottles of agar at just
the right temperature. (Why?)
• The spread plate method
– Advantage:
• is easier to do
– Disadvantage:
• may give an uneven lawn.
Testing
• Testing may be achieved either by either
of 2 methods:
- placing antibiotic liquid into wells or
ditches which have been cut into the
agar, or
- applying discs containing measured
amounts of antibiotics, which will diffuse
out.
Testing bacteria for sensitivity to
antibiotics
• Individual bacterial strains can be tested
against a variety of antibiotics (or vice
versa) by growing the bacteria as "lawns"
on agar in the presence of different
concentrations of a single antibiotic, or
several different antibiotics may be
tested at the same time.
• Under what circumstances might individual bacterial
strains be tested against a variety of antibiotics?
–To see if a bacterium causing an infection can be
controlled by a specific antibiotic/find the best one
(for a particular patient)
•Under what circumstances might individual antibiotics
be tested against a variety of bacterial strains?
–To see if an antibiotic under development by a drug
company is likely to work in a given circumstance - i.e.
control a certain disease
• The size of the zone of inhibition - in
which bacteria will not grow - gives an
indication of the sensitivity of the
strain involved, i.e. how easily the
bacterial strain will be controlled by
the particular antibiotics.
• In a medical context, the prescription by a
doctor of an appropriate antibiotic or dose
rate for a particular patient may thus be
confirmed from these laboratory tests.
• How long would these laboratory tests
take? Explain why.
About 24 hours (or at least overnight) - to
give the culture a chance to grow (or be
killed!)
• What is the difference between a
disinfectant and a chemotherapeutic
agent?
• What is meant by the following?
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Therapeutic dose
Toxic dose
Therapeutic index
Broad spectrum antibiotic
Narrow spectrum antibiotic
Minimal inhibitory concentration
Minimum lethal concentration