Site author Richard Steane
The BioTopics website gives access to interactive resource material, developed to support the learning and teaching of Biology at a variety of levels.

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.

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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

This testing may be achieved either by either of 2 methods:
- placing antibiotic liquid into wells or ditches which have been cut into the agar (using a heat-sterilised corkborer), or
- (using heat-sterilised forceps) applying discs containing measured amounts of antibiotics, which will diffuse out.

Similar techniques can be used to compare the effectiveness of different antiseptics and disinfectants.

bacterial sensitivity testing

What is meant by the term bacterial lawn?
> an even layer of bacteria growing on the surface of /within an agar plate

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.

By measuring the diameter of these circular zones, their cross-sectional area can be calculated using the expression πr2 where r = ½ of the diameter (and π = 3.142).

In a medical context, the prescription by a doctor of an appropriate antibiotic or dose rate for a particular patient suffering from a bacterial infection 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!)
And there is the lag time for the delivery of the specimen to the pathology lab, and the reporting process.

This topic has connections with other units on this site:

Production of Penicillin in Industrial Fermenters (Bioreactors) - This topic used to be included in this file
Antibiotics Some basic information
Types of antibiotics Slightly more detail, covering brief historical introductions, naming key people where possible as well as chemical information on their molecular structure and mode of action, backed up with several links to interactive 3D molecule displays

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