Antimicrobial Susceptibility Testing

Question 1

Disk Difussion inoculom

Answer 1

1.5 x 10^8/cfu

Question 2

Agar Dilution inoculum

Answer 2

1x10^4/spot

Question 3

Broth dilution inoculum

Answer 3

5.0x10^5/cfu

Question 4

Standard

Answer 4

0.5% Macfarland Standard

Question 5

Preparation McFarland

Answer 5

0.5 ml of 1.175% BaCl
99.5 ml of 1% H2SO4

Question 6

Standard Medium

Answer 6

MHA

Question 7

pH

Answer 7

7.2-7.4

Question 8

Agar depth

Answer 8

4 mm or 3-5 mm

Question 9

Time it takes for drying of inoculum

Answer 9

< 5 minutes

Question 10

Time it takes for disk diffusion

Answer 10

3-5 minutes (<15 minutes)

Question 11

Too thick

Answer 11

False Resistant - small ZOI

Question 12

Too thin

Answer 12

False sensitive - LARGE ZOI

Question 13

Low pH

“LOW CAE”

Answer 13

Decreased activity:
Clindamycin
Aminoglycoside
Erythromycin

Question 14

High pH

Answer 14

Decreased acitivity: Tetracycline

Question 15

High Thymidine

Answer 15

False Resistant - Sulfonamide, Trimethoprin

Question 16

High Ca, Mg

Answer 16

Decreased aciivity of Aminoglycosides (P. aeroginosa) and Tetracycline (all organisms)

Question 17

Zone within a zone phenomenon

Answer 17

Swarming Proteous

Question 18

Inoculum to heavy

Answer 18

Small ZOI (FR)

Question 19

Inocul

Inoculum to light

Answer 19

Large ZOI (FS)

Question 20

Inoculated plates incubated in CO2 incubator

Answer 20

CO2 diffuse into the medium, causing the formation of carbonic acid, which will lower th pH of medium

Question 21

Susceptibility testing performed on quality control organisms using a new media lot number yielded zone sizes that were too large for all antibiotics tested. The testing was repeated using media from a previously used lot number, and all zone sizes were acceptable. Which of the following best explains the unacceptable zone sizes?

Answer 21

Depth of the media was too thin

Question 22

In a disk diffusion susceptibility test, which of the following can result if disks are placed on the inoculated media and left at room temperature for an hour before incubation?

Answer 22

Zones of larger diameter would result
A delay of more than 15 minutes between placing the disks on an inoculated plate and incubation permits excess prediffusion of the antimicrobial agent from the disk.

Question 23

In breakpoint antimicrobial drug testing, interpretation of susceptible (S), intermediate (I), and resistant (R) refers to testing antibiotics by using:

Answer 23

Only the specific concentrations necessary to report S, I, or R