7. Antimicrobial Susceptibility Tests Flashcards
What is universal susceptibility?
It is adequate enough to identify the organism and not perform a susceptibility test because it is known that the organism is sensitive to an antimicrobial agent
What is the purpose of susceptibility testing?
To detect the emergence of resistant strains and to determine which antimicrobial would be best used to kill or control an organism causing an infection. Which antimicrobials would be effective.
What is in vivo vs. In vitro?
In vitro - test tubes
In vivo - in the living body
What are the three methods for performing susceptibility tests?
Broth dilution
Agar dilution
Disc diffusion
What are some considerations for susceptibility tests?
There is a standard bacterial inoculum - 0.5 McFarland standard 1.5 x 10^8 CFU/mL
3-10 colonies should be sampled
Standard inoculum should be in log growth phase
What is the macro broth dilution method?
The antimicrobial being tested is placed in a series of dilutions into test tubes and the organism being tested is added to each tube. The tubes are incubated then inspected for visible growth and from this the concentration of antimicrobial required to inhibit bacterial growth can be determined. The last tube does not get antimicrobial and serves as a growth control. Measured in SI units mg/L
SEE PAGE 7 OF MODULE
What is the minimal inhibitory concentration?
The lowest concentration of antimicrobial in mg/L that prevents in vitro growth of the infectious organism
The in vivo level should be 2-4 times higher than the in vitro MIC
What is the minimum bactericidal concentration?
Is the lowest concentration of antimicrobial in mg/L that results in more than 99.9% infectious organism death (MBC)
Not usually required for effective treatment, and MIC must be done first
What are microdilution broth susceptibility tests?
The antimicrobials being tested are placed as a series of dilutions into small micro tube wells and the organism being tested is added to each well. The plates are incubated then inspected for visible growth and from this the concentration of each antimicrobial required to inhibit bacterial growth can be determined (MIC). The test plate also contains a sterility well and a growth control well. Measured in SI units mg/L.
Frozen -20 degrees, lyophilized 4 degrees, used for gram positive, gram negative, urine
SEE PAGE 15 FOR DIAGRAM
What are some automated systems for the microdilution broth susceptibility test?
The Vitek, Vitek 2
Uses microwell plastic cassettes or cards, checks turbidity
What are purity plates?
To subculture standardized inoculum to appropriate media to check for purity. Plates must be observed on all tests for purity before patient results can be released.
What are agar dilution susceptibility tests?
Same as the broth dilution method except the antimicrobials are diluted in agar instead of broth
Use Mueller Hinton agar, inoculum is standardized
(Steers replicator pin inoculation)
Sometimes used to screen for resistant strains
Can adapt to fastidious organisms (need special nutrients)
What is the disc diffusion susceptibility test/Kirby Bauer sensitivity test?
Standardized inoculum is spread over the surface of an agar plate and antibiotic discs are placed on the surface. Plate is incubated and the antimicrobials diffuse into the agar to form a circular zone around the disc, colonies do not form where there is enough antimicrobial to inhibit growth (called the zone of inhibition). The zone is then measured and used to interpret the sensitivity.
What causes false results in the Kirby Bauer sensitivity test?
Inoculated over 4-6 hours = falsely sensitive results
Discs must be placed on plate within 15 minutes - falsely resistant smaller circle
Plates must be incubated within 15 minutes of placing discs - larger circle, falsely sensitive
Must be in log growth phase
Thin agar gives a false large diffusion circle
Thick agar gives a false small diffusion circle
Surface moisture
pH
Stiff agar slows diffusion giving a false small circle
What is the critical point?
Lowest concentration of antimicrobial around the disc that inhibits growth
