AST Flashcards
What is the purpose of Antimicrobial Susceptibility Testing (AST)?
To determine the susceptibility of bacteria isolated from clinical samples causing infectious disease
Susceptibility is often unknown prior to testing.
What standard is used for inoculum preparation in AST?
0.5% McFarland Standard
Inocula should be used within 15 minutes of preparation.
What does the Dilution Method in AST determine?
Minimum inhibitory concentration (MIC)
MIC is the lowest concentration of an antimicrobial that inhibits bacterial growth.
What is the difference between macrodilution and microdilution methods?
Macrodilution uses larger volumes, while microdilution allows testing of multiple antimicrobial agents at once
MIC is measured using a tray reading device in microdilution.
What does the term Minimum Bactericidal Concentration (MBC) refer to?
The lowest concentration that reduces 99.9% of colonies from subculture after MIC test
Particularly important for immunocompromised patients.
Define Susceptible (S) in the context of AST.
Bacterial infection should respond to the antimicrobial agent.
What does Resistant (R) indicate in AST results?
Bacterial infection will not respond to the antimicrobial agent.
What does Intermediate (I) mean in AST?
Microorganism is in a range of susceptibility where MIC approaches/exceeds the level of antimicrobial agent ordinarily achieved.
What is Non-susceptible (NS) in AST?
Isolates with established susceptible interpretive criteria that fall above/below susceptible breaking points.
What is Susceptible-Dose Dependent (SDD)?
Within the intermediate category, susceptibility depends on the dosing regimen.
What does a positive B-lactamase test indicate?
Production of B-lactamase, leading to resistance to B-lactam antibiotics.
What is the significance of Extended Spectrum Beta-lactamases (ESBL)?
Organisms produce ESBL which can destroy the activity of certain antibiotics
ESBL-producing organisms are resistant to penicillins, cephalosporins, and aztreonam.
What does the D test screen for?
AmpC enzymes which confer resistance to cephalosporins, penicillins, and aztreonam.
What are the main features of the Vitek System in automated AST?
Determines MIC in 4-10 hours using kinetic measurements to analyze growth curves.
What type of medium is used for testing Haemophilus spp.?
Haemophilus test medium (HTM).
How is MRSA identified in the laboratory?
Using Oxacillin with a standardized inoculum on an Oxacillin screen plate.
What is the role of serum bactericidal test (SBT)?
To determine bactericidal and bacteriostatic parameters.
What are the conditions for incubating anaerobes in AST?
Incubation in broth microdilution with Brucella broth + lysed horse blood at 35°C for 48 hours.
What is the function of carbapenemases?
Plasmid-mediated enzymes that confer resistance to all penicillins, cephalosporins, carbapenems, and aztreonam.
What is meant by quality control in AST?
Testing reference strains of bacteria with known antimicrobial susceptibility.
What factors limit the method of testing in AST?
The method selected for testing and the patient population considered.
