Antimicrobial Susceptibility Testing PART 3

Question 1

Broth Dilution:

- Inoculum prep

Answer 1

3-5 colonies and make a McFarland standard

Question 2

Broth Dilution:

- McFarland Standard

Answer 2

Regular 0.5 McFarland diluted down to a concentration of 5x10^5 CFU/ml

Question 3

Broth Dilution Mirotiter Tray:

- Incoculation

Answer 3

• Deliver inoculum, via delivery prongs, into sterility tray with sterility well (neg) and growth control well (pos)
• make a PURITY PLATE

Question 4

Broth Dilution Mirotiter Tray:

- Incubation

Answer 4

• ambient air
• 16-20 hours
• 35C

Question 5

Broth Dilution Mirotiter Tray:

- Quantitative or Qualitative

Answer 5

Quantitative because you report a # AND you look as CLSI table to interpret S, I or R

Question 6

Broth Dilution:

- Reading results

Answer 6

MIC is lowest concentration that inhibits growth

- needs to be reproducible with one well margin of error

Question 7

SXT is read at what % inhibition

Answer 7

80%

Question 8

MIC/MBC procedure

Answer 8

Take clear tubes from MIC and incubate again on a plate. If < 50 colonies can grow on a plate thats considered killed!

Question 9

Minimal Bactericidal Concentration

Answer 9

killing capability, 99.9% of original inoculum

Question 10

Minimal Inhibitory Dilution (MID) procedure

Answer 10

patients serum is serially diluted and a standard inoculum (5x10^5 CFU/ml) of patient organism is added. Incubate and read highest dilution which inhibits growth.

Question 11

Minimal Bactericidal Dilution (MBD)

• What is MBD
• What are the results

Answer 11

• Highest dilution of patient serum that kills 99.9% of an original inoculum of patients organism
• results are dilutions 1:8 to 1:32 is adequate

Question 12

What is the reference method for susceptibility testing?

Answer 12

Agar Dilution

Question 13

Agar dilution set up

Answer 13

Antibiotic diluted into agar in two-fold concentrations

Question 14

Agar Dilution used for what type of organisms

Answer 14

Fastidious organisms (Neisseria gonorrhoeae)

Question 15

Agar Dilution, Steers replicator:

- standard inoculum per spot =

Answer 15

10^4 CFU/ml

Question 16

Agar Dilution, Steers replicator:

- incubation requirements

Answer 16

• ambient air
• 35C
• 18-20 hours

Question 17

Automated Methods: DADE MICROSCAN

- three methods

Answer 17

1. Traditional MIC methods
2. Photometric Reading (turbidity)
3. Fluormetric Reading (best method)

Question 18

Automated Methods: BD PHOENIX SYSTEM

- 3 methods

Answer 18

1. Gravity-Based inoculation process
2. Redox indicator system
3. Data management system

Question 19

Automated Methods: VITEK

- 3 methods

Answer 19

1. Computer assisted analysis of growth
2. Algorithm derived MIC
3. Calculated MIC

Question 20

Three methods for beta-lactamase detection

Answer 20

1. Chromogenic cephalosporin: Cefinase, Nitrocefin
2. Acidometric
3. Iodometric

Question 21

Results of the Beta Lactamase Test

Answer 21

Positive = organism produces enzyme to degrade penicilin, ampicillin, and amoxicillin 
Negative = does not produce enzyme but it may have some other mechanism of resistance to the same drug

Question 22

What is the value of measuring serum levels ?

Answer 22

Want to know how much antibiotic is at site of infection so that therapeutic range can be determined