Antimicrobial Susceptibility Testing - Testing Procedures and Automation

Question 1

Objective 1: **Recognize the antimicrobial agent still universally susceptible for Streptococcus
pyogenes isolates

Answer 1

Penicillin

Question 2

Objective 2: **Define Breakpoint

Answer 2

A value to categorize an microorganism in the presence of an antimicrobial agent

Question 3

Objective 2: **Define Minimal Inhibitory Concentration (MIC)

Answer 3

Lowest amount of antimicrobial agent that prevents 100% of visible growth of an organism

Question 4

Objective 2: **Screening Test

Answer 4

Provides presumptive results, requires confirmation

Question 5

Objective 2: **Surrogate Agent Test

Answer 5

One agent replaces another agent for testing

Question 6

Objective 3: List factors to consider for the detection of resistance by laboratory methods.

Answer 6

• Test significant pathogens
• Detects the most common types of resistance
• Phenotypic testing methods
• Genotypic testing methods

Question 7

Objective 4: Identify the types and number of interactions that may occur between patient,
organism, and antimicrobial agent

Answer 7

Drug to Bug
- Drug: Kills or inhibits growth
- Bug: Permeability, Alteration of Target Site, Enzyme
inactivation/degradation

Bug to Patient
- Bug: Toxins, adhesion, biofilms
- Patient: antibodies, phagocytosis, etc

Patient to Drug
- Patient: Metabolize, excrete, concentrate, etc.
- Drug: Toxicity?

Question 8

Objective 5: **Explain the principles for standardization of susceptibility testing including: growth medium, pH, cation concentration, incubation conditions, and inoculum density.

Answer 8

Growth medium:
- MH agar/broth
- Depth: 3-5 mm

pH:
- 7.2 to 7.4

Cation Conc.
- Mg, Ca, NaCl

Incubation:
- Ambient Air @ 35-37 C

Inoculum density:
- Dependent on testing methods

Question 9

Objective 11: Identify the advantages and disadvantages of diffusion and dilution susceptibility
testing

Answer 9

Dilution test:
Advantages
- Quantitatve results
- Reproducibility
- Automation
Disadvantages
- Inflexiblity of drug selection for testing

Diffusion test:
Advantages
- Simplicity
- Reproducibility
- Categorical results
- Flexibility of drug selection for testing
Disadvantages
- Lack of automation

Question 10

Objective 13: Recognize principles of automated methods for susceptibility testing (TREK, Microscan,
Phoenix, and Vitek)

Answer 10

Common thread: all of the automated systems use photometric/light detection of the growth to determine susceptibility that is based on the broth dilution technique

Question 11

Objective 14: Explain the value of measuring serum antimicrobial levels

Answer 11

Avoid toxicity in the patient – narrow therapeutic range

Question 12

Objective 15: List antimicrobial agents typically measured for toxicity levels in the clinical laboratory

Answer 12

Vancomycin
Gentamicin
Tobramycin
Amikacin