PC2 1.1.6 Bacteria Susceptibility Tests

min. inh. conc. (MIC)

inh. visible growth in 24H

Can you compare the MICs of different drugs? Why/why not?

no, they're individually unique

min. bactericidal conc. (MBC)

cell death by 1k fold in 24H

bacteriostatic

kills at slower rate

MBC/MIC ratio values

≤4 - bactericidal

>4 - bacteriostatic

List the qualitative testing method & how it's performed

disk diffusion (kirby-bauer) - eval. diameter

What 3 things can impact results of a disk diffusion test?

1) antibiotic amt & properties

2) bacteria conc.

3) susceptibility

What are the categories of susceptibility? (3)

susceptible

intermediate (SDD)

resistant

Why does the intermediate (susceptible dose-dependent) category exist?

technical variability (combo therapy, inc. dose/infusion time)

T/F: Quantitative methods produce the exact MIC

true

List the quantitative testing methods & how they're performed (3)

1) broth/agar macrodilution - 2 dilution

2) e-test - gradient dilution strips

3) automated systems - colorimetric wells

A 2 dilution for macrodilution means there is _____ concentration of antibiotic in the tube compared to the previous tube.

one-half

In macrodilution, the first _____ that lacks growth is the MIC while the first _____ that lacks growth is the MBC.

tube

plate

______ dilution is the gold standard for MIC, but is time consuming & is only used for research.

Micro

E-tests can also be used for __________ detection.

resistance

Aside from testing susceptibility, automated systems can also ___________ organisms.

identify

What are the disadvantages of using automated systems? (2)

1) over/under inoculation

2) limited range of organisms (no fastidious)

breakpoints

serum drug conc. for best success

What organizations created across the board standards for breakpoints (CURES ACT)? (2)

CLSI

FDA

The CURES ACT was created using... (3)

1) PK/PD data

2) clinical outcomes

3) epidemiological cut offs (bell curve)

The _______ dilutions under breakpoint, the better the antibiotic.

more

What will determine which drug to use if they have the same number of dilutions under breakpoint?

broad vs. narrow spectrum activity

T/F: You should avoid using broad spectrum therapy if a narrow option is available.

true