Antibiotic Sensitive Testing
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In the past, how were infectious diseases treated?
empirically based on expectation that the bacteria is susceptible to the drug
How are infectious diseases treated now?
by selecting an appropriate antimicrobial agent (which can be challenging)
Is the resistance to essentially all antimicrobials been documented?
What is susceptibility testing?
- determines susceptibility of the bacteria to an antimicrobial drug
- determines resistance of the bacteria to an antimicrobial drug
What do the results of a susceptibility test indicate?
which antimicrobials to use
Should the specimen used for testing be taken prior to or after any treatment with antimicrobials?
Can antimicrobials be given once the sample has been taken and before we get the results back from the test?
yes and then once the results are in the antimicrobial can be changed to a more appropriate one based on the test results
Which is better to use for a susceptibility test, an isolated pure culture or a fresh sample?
an isolated pure culture
Why is it better to use an isolated pure culture when doing a susceptibility test?
ensures that your sample is pure and not mixed
How do we determine which antibiotic discs would be more effective?
gram stain the isolated culture
Which agar do we use for the Kirby-Bauer method? Why?
Mueller-Hinton agar because it is the most reproducible, standardized zone of inhibition results
What is the Kirby Bauer method?
paper discs impregnated with antimicrobials
What does the concentration of the antimicrobial drug in each disc correlate with?
blood and tissue levels in the patient
What are the steps to doing the Kirby Bauer method?
- bacteria is swabbed on the agar to make an even growth (lawn)
- antimicrobial discs are placed on the surface of the agar
- antibiotic diffuses from the disc into the agar
- incubate for 18 - 24 hours
- look at the zones of inhibition
What is the zone of inhibition?
no growth of bacteria in the immediate area surrounding the disc (clear zones)
What causes the zones of inhibition?
bacteria have been killed or inhibited from growing by antibiotic which has diffused into the agar from the disc
What does the size of the zones of inhibition depend on?
the sensitivity of the bacteria to the antibiotic and to the rate of diffusion of the antibiotic through the agar
What are the three different results from the zone of inhibition?
How do we read the plates to get the zone of inhibition?
- hold the plate up to the light
- measure from the back with a ruler
- measure at the widest diameter
- measuring units are mm
If the zone of inhibition is sensitive, what does that mean?
bacteria may be treated with standard dose of antibiotics
If the zone of inhibition is intermediate, what does that mean?
infection is sensitive if treated with higher than standard doses, or in areas that concentrate the antibiotic
If the zone of inhibition is resistant, what does that mean?
infection is not inhibited by the usual dose of the drug - not likely to be effective
Should the be moisture in the lid or on the agar when doing a sensitivity test?
Once we spread the culture on the plate, what should we do before adding the discs?
keep dry, covered, and at room temperature for no more than 15 minutes
How should the discs be stored when not using them?
in the refrigerator
What should we use to place discs on the agar for an antibiotic sensitivity test?
dispenser or sterile forceps
Should we compare zone sizes of different discs directly with each other? Why or why not?
- some drugs do not readily diffuse through the agar and therefore will give smaller zones of inhibition
- always compare to the standard
What are the different combination plates for the bullseye urine plate?
- selective media
- differential media
What are the 5 zones for the bullseye urine plate?
- 1 - EMB agar
- 2 - XLD agar
- 3 - citrate agar
- 4 - TSA blood agar
- center - mueller-hinton agar
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