AST Exam Flashcards
Resistance from genetically encoded traits; can be Intrinsic or Acquired
Microorganism mediated resistance
Resistance resulting from physical or chemical characteristics of the environment
Environmentally mediated resistance
Change such that a drug is no longer suitable for clinical use
Clinical resistance
Change in cell physiology or structure that result in observably reduced susceptibility to an antibiotic (acquired)
Biologic resistance
Resulting from structural or physiologic state of a microorganism; Present long before antibiotic usage by humans
Intrinsic Resistance
Concern for infection control; Due to modification of genetic makeup of organism; Organism develops resistance to antibiotic to which it was previously susceptible
Acquired Resistance
What may cause acquired resistance?
Spontaneous chromosomal mutation; Transfer of genetic material - transformation, transduction, or conjugation
What are common sources of resistance genes?
Plasmids and transposons
Name the resistance mechanism: Especially for Beta lactams and Aminoglycosides
Enzyme Degradation
Name the resistance mechanism: Every class of antibiotics has this type of resistance
Altered target
Name the resistance mechanism: Especially important for Gram negatives!
- Porin loss or alteration
- Efflux mechanism
Decreased Uptake – membrane permeability
- All cleave the Beta lactam ring
- Likely emerged before human use of beta lactam antibiotics
β-lactamases
What Beta- lactamase does this combo inhibit? Augmentin (=Amoxicillin + Clavulanate)
Clavulanic acid
What Beta- lactamase does this combo inhibit? Ampicillin-Sulbactam
Sulbactam
What Beta- lactamase does this combo inhibit? Timentin (=Ticarcillin + Clavulanate)
Clavulanic acid
What Beta- lactamase does this combo inhibit? Piperacillin Tazobactam “Pip Taz”
Tazobactam
What’s the goal/purpose of an AST?
- Test for acquired resistance (organism based) to therapeutic agents
- Establish antimicrobial susceptibility profiles
Why standardize AST?
Optimize bacterial growth conditions, Optimize conditions for maintaining antimicrobial integrity, Maintain reproducibility and consistency
Why optimize bacterial growth conditions for AST?
To ensure that the inhibition of growth can be attributed to the antimicrobial agent.
Why optimize bacterial growth conditions for AST?
To ensure you are attributing the failure to inhibit bacterial growth to organism-associated resistance.
What are the standardizing components for growth medium of AST?
pH, cation concentration, blood/serum supplements, thymidine content
What are the standardizing components for incubation of AST?
Atmosphere, temperature, duration
What are the other two standardizing components for AST (besides growth medium and incubation requirements)?
Inoculum size and antimicrobial concentration
Do these methods directly detect a resistance mechanism or measure antibiotic activity?
- Conventional - broth dilution, agar dilution, disk diffusion
- Commercial systems
- Special screens & indicator tests
Directly measure antibiotic activity
Do these methods directly detect a resistance mechanism or measure antibiotic activity?
- Beta-lactamase test
- PCR testing for resistance genes
Directly detect a resistance mechanism
What is the turbidity standard for AST?
0.5 McFarland turbidity standard (1.5 x 10^8 colony-forming units (CFUs) per milliliter)
How is standard turbidity achieved?
“Eyeballing it” in comparison to McFarland standard or using a turbidity meter
What is a broth dilution used for in AST?
Challenges the organism of interest with an antimicrobial agent in a liquid environment.
What are the two types of broth dilutions?
- Microdilution—Total volume is 0.05 to 0.1 mL (microtiter tray)
- Macrodilution—Total volume is 1 mL or greater (tubes)
How are broth dilution tests reported?
Reported in µg/mL
What kind of dilution is commonly used for broth dilution methods?
A series of doubling dilutions
Which is the preferred method of broth dilution?
Microdilution
What are some of the reasons microdilution is preferred?
Macrodilution is too labor intensive; Microdilution is commercially prepared, automated or semi-automated, and you can run multiple drugs at once
The well with the lowest concentration of antimicrobial agent that inhibits growth
Minimum Inhibitory Concentration (“MIC”)
How do we grade organisms as a result of MICs?
Susceptible, Intermediate, Resistant, Susceptible/Dose-dependent
What is a breakpoint? Who establishes them?
Specific concentrations that define the different categories of the MIC; CLSI
What causes skipped wells?
Contamination
From an agar dilution how are MICs determined?
The MIC is the lowest concentration that inhibits visible growth.
Name the AST method described? Antibiotic disks of known concentrations are placed on the surface of an agar plate that has been seeded with a lawn of bacteria (0.5 McFarland turbidity standard).
Disk Diffusion – Kirby Bauer