1. 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.

2. 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.

3. 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.

4. 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.

5. 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
is easier to do
may give an uneven lawn.

6. 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.

9. 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.

10. 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

11. 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.

12. 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!)

13. What is meant by the following?
What is the difference between a disinfectant and a chemotherapeutic agent?
What is meant by the following?
Therapeutic dose
Toxic dose
Therapeutic index
Broad spectrum antibiotic
Narrow spectrum antibiotic
Minimal inhibitory concentration
Minimum lethal concentration