ANTIMICROBIAL SUSCEPTIBILITY TESTING Flashcards
memorization
It is the least amount of antibiotic (highest dilution) that prevents visible growth.
Minimal Inhibitory Concentration (MIC)
Concentration of antibiotic that results in 99.9% decrease in CFU per mL
Minimal Bactericidal Concentration (MBC)
Kirby-Bauer disk diffusion test:
Principle: There is a/an ________ relationship the diameter of the zone of inhibited growth around the antibiotic disk and the logarithm of the ____________ of the antibiotic
There is an INVERSE LINEAR relationship;
the logarithm of the MINIMAL INHIBITORY CONCENTRATION
The lower the MIC, the ____ potency, and the ________ zone of inhibition
The lower the MIC, the higher potency, and the larger the zone of inhibition
Plate size used in Kirby-Bauer disk diffusion test:
150 x 15 mm plate
Agar used in KB disk diffusion test:
Mueller-Hinton Agar
pH of the medium should be:
7.2 to 7.4 pH
Agar depth should be held constant at:
4 mm (if in range: 4-6 mm)
The density of the inoculum should be adjusted to:
0.5 McFarland Standard
Equivalence of McFarland standard into CFU/mL
1.5 x 10^8 CFU/mL
if in range: 1-2 x 10^8 CFU/mL (Recall)
Preparation of McFarland Standard:
0.5 mL of 1.175% BaCl2 and
99.5 mL of 1% H2SO4
Too heavy inoculum results in:
Smaller zones/ False resistant
Too light inoculum results in:
Larger zones/ False susceptible
Only overnight colonies less than ____ hours should be test.
24 hours
Let inoculum dry at least ______ but not more than ______ before adding the disks.
at least 5 minutes but not more than 30 minutes
Too much moisture on the surface of agar results in:
Confluent growth, smaller zone, FR (false resistant)
Very dry agar surface results in:
Gives poor growth, larger zone, FS (false susceptible)
Place no more than ____ disks per 150-mm plate (___ disks in outer ring, ___ in center)
Place no more than 12 disks per 150 mm plate (8 disks in outer ring, 4 in center)
Distance between the disk is:
24 mm
Antibiotics that are best indicators of poor storage, as they are the first to deteriorate
- Penicillin
- methicillin
Incubate plate at:
35C for 16-18 hours
If in range: 35 +/- 2C (Recall)
Incubation of MRSA may require:
24 hours
Incubate the plate in an _______ position
UPSIDE DOWN
Why are plates incubated in an Upside-down position?
To avoid accumulation of moisture
Quality control:
Test QC strains of ______, ______, _______, _______ for _______ days.
Test QC strains of:
E. coli
S. aureus
P. aeruginosa
Enterococcus faecalis
for 20-30 consecutive days.
If QC results are acceptable, frequency can be reduced to ______ basis.
weekly
Three (3) types of B-lactamase direct tests:
- Acidometric
- Iodometric
- Chromogenic
Most sensitive, most specific, and easiest B-lactamase test to use:
Chromogenic
What is the positive color of the chromogenic B-lactamase test?
RED
Plastic strip containing antibiotic concentration gradient placed on inoculum lawn on Mueller-Hinton plate & incubated overnight. It is used to determine MIC:
E test
A test used to detect inducible clindamycin resistance MRSA isolates that are resistant to erythromycin & susceptible to clindamycin on initial testing.
D test
