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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)

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bacterial lawn
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
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
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
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 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
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?

Therapeutic dose

Toxic dose

Therapeutic index

Broad spectrum antibiotic

Narrow spectrum antibiotic

Minimal inhibitory concentration

Minimum lethal concentration