Antimicrobial Susceptibility Testing Flashcards
What are the 2 main purposes of susceptibility testing ?
- treatment guide
2. epidemiological tool (ie tracking resistance)
What are the 7 components in susceptibility testing
- ID of organism
- ID of infection site
- Selection of antibiotics
- Selection of appropriate test method
- Interpretation
- Selective reporting
- Quality control
What 4 things do you want to look for when you are determining the organism?
- Predictably susceptible?
- Intrinsic resistance
- Inducible resistance
- Hetero-resistance phenotypes
What are ß-haemolytic strep never resistant to ?
ß-lactams
What drug are gram negative bacteria intrinsically resistant to?
Vancomycin
What drug are the enterococci intrinsically resistant to?
Cephalosporins
What considerations need to be taken in relation to site of infection?
if in the brain, need a drug that will cross the blood/brain barrier
If you have a UTI you need a drug that will be excreted renally vs hepatically
If you have an RTI you cant use Daptomycin
What factors ensure the quality control of the testing?
Reference strains (ATCC)
- MIC ranges / zone diameters
Media - pH, depth, thymidine content, divalent cation content
Antibiotics - shelf life, potency
Incubation temp, time, atmosphere, inoculum
What does the nitrocephin disk test for?
Tests for ß-lactamase production
What is MIC?
Minimum Inhibition Concentration (MIC) = The lowest concentration of antimicrobial agent that inhibits bacterial growth/ multiplication
What is another name for the disc-diffusion method?
The Kirby-Bauer test
How does the Kirby-Bauer/Disc-diffusion test work?
Antibiotic-impregnated filter disc
Susceptibility test against more than one antibiotic by measuring size of “inhibition zone ”
- gives you a qualitative result that is comparable to quantitative results from MIC tests
What 10 factors can affect the size of the zone of inhibition
- Inoculum density
- Timing of disc application
- Temperature of incubation
- Incubation time
- Depth of agar
- Spacing of discs
- Potency of antibiotic discs
- Composition of media
- pH of media
- Reading of the zones (subjective errors)
How does the inoculum density affect the ZoI size?
lighter inoculum density will give you larger zones and vice versa
How does the timing of disc application affect the ZoI size?
If after application of disc, the plate is kept for longer time at room temperature, small zones may form
How does the temperature affect the ZoI size?
Larger zones are seen with temperatures < 35ºC
what is the ideal incubation time?
16-20 hours
What is the ideal depth of agar? how does deviation from this affect the ZoI ?
4 mm
Thin media yield excessively large inhibition zones and vice versa
What zones of inhibition (for certain drug types) are larger in acidic pH ?
Tetracycline, novobiocin, methicillin zones are larger
What zones of inhibition (for certain drug types) are larger in basic pH ?
Aminoglycosides, erythromycin
How does double diffusion disk test work for the macrolide resistance phenotypes?
You have a clindamycin (lincosamide) and a macrolide disk near each other on a dish
- inducible MLS phenotype will show the “D test” - blunting of the ZoI
- constitutive MLS phenotype will show no ZoI around either disk
- M phenotype. Big ZoI around the clindamycin, small to none around the macrolide
How do you read the MIC from an E test?
The point (read from scale) where the zone of inhibition intersect the strip
What is the MBC?
Minimum Bactericidal Concentration (MBC) or Minimum Lethal Concentration (MLC) = The lowest concentration of antimicrobial agent that allows less than 0.1% of the original inoculum to survive
How do you determine MIC and MBC from broth dilution?
for MIC you look for turbidity so the first dilution at which you can see any growth with the naked eye
For MBC you subculture all the non-turbid tubes and see at which concentration you have <0.1% growth
What’s the procedure for agar dilution?
Using a replicating inoculator device called “A Steers-Foltz replicator” deliver 0.001 ml of bacterial inoculum into an agar with known concentration of antibiotic
Put a bunch of different bacteria onto a single plate
What is the name for the automated system of susceptibility testing ?
Vitek
