Antimicrobial Susceptibility Testing Flashcards
Common Abuses of Antimicrobial Susceptibility Testing (AST)
- Using non-standard test methods
- AST is an in vitro standardized test that specifies:
- Test medium:
- pH
- Cation concentrations
- Incubation:
- Temperature
- Time
- Atmosphere
- Bacterial inoculum
- Antimicrobial Concentration
- Performance of Quality Control testing
- Test medium:
- AST is an in vitro standardized test that specifies:
- Changing the test conditions may make the antibiotic appear more or less active in vitro which could lead to false “susceptible” results
AST in the Laboratory:
Broth Dilution Testing
AST test performed in broth media
Uses a Log2 series of antimicrobial concentrations
Microwell or macro-broth
- Test a pure culture of the isolate
- Bacterial inoculum = 0.5 Mcfarland ~ 5 X 105 CFU / mL
- Incubate for 16-20 hrs at 35C in room air
- Tested in cation-adjusted Mueller-hinton broth
- Routine QC weekly
Minimal Inhibitory Concentration
- “the lowest concentration of an antimicrobial agent that prevents visible growth of a microorganism in an agar or broth dilution susceptibility test”
AST in the laboratory:
Disk Diffusion Testing - Kirby-bauer
- Bacterial inoculum - 0.5 McFarland - Lawn growth
- Standardized disk strength
- Diffusion creates a “gradient” of antimicrobial concentrations
- Test result = zone of inhibition
Which Method is Better?
Broth dilution vs. disk diffusion
- Broth dilution:
- automation
- More drugs with veterinary interpretive criteria
- MIC
- clinically useful for dosing regimen modification
- Disk Diffusion:
- more flexible test options
- Cheaper test to conduct
Breakpoints:
The MIC or Zone of inhibition is linked to expected clinical outcome through breakpoints / interpretive category
Breakpoint:
Minimal inhibitory concentration or zome diameter value used to indicate susceptible, intermediate, and resistant
Interpretive Categories:
Susceptible
category that implies that an infection due to the strain may be appropriately treated [in that animal species] with the dosage regimen of an antimicrobial agent recommened for that type of infection and infecting species
Interpretive Categories:
Intermediate
Category that implies that an infection due to the isolate may be appropriately treated in body sites where the drugs are physiologicdally concentrated or when a high dosage of drug can be used
Also the “buffer zone” that should prevent small, uncontrolled, technical factors form causing major discrepancies in interpretations
Interpretive Categories:
Resistant
Strains [which] are not inhibited by the usually achievable concentration of the agent with normal dosage schedules and / or fall in the range [of MICs/ZOIs] where specific resistnace mechanisms are likely, and clinical outcome has not been predictable in effectiveness studies
Interpretive Categories
Veterinary approved interpretive categories are NOT just specific toa n antibiotic
The “S”, “I”, “R” for that antibiotic should be interpreted in context of:
Animal species
Dosing regimen
Antimicrobial
Type of infection
Bacterial species
Developing Breakpoints
Breakpoints are developed by the CLSI-VAST based on:
Wild type cutoff COwt
Parmacokinetic / Pharmacodynamics cutoff COpk/pd
Clinical cutoff COcl
Developing Breakpoints:
Wild-Type Cutoff - COwt
Separeate isolates that do not have resistance elements ( wild-type) form isolates that do possess resistance genes based on phenotype
Developing Breakpoints:
Pharmacokinetic / Pharmacodynamic Cuttoff - COpk/pd
Considers the absorption, distribution, metabolism, and elimination (and the vairability of these parameters within a population of patients) of an antibiotic and the necessary concentrations of a drug to treat the infection
Developing Breakpoints:
Clinical Cutoff - COcl
Evaluates the clinical response of animals with a specific disease syndrome and isolates with various MICs to the antibiotics
Developing Breakpoints
Decision Tree
