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.
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.
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.