Antimicrobial Susceptibility Testing Flashcards
Disk Difussion inoculom
1.5 x 10^8/cfu
Agar Dilution inoculum
1x10^4/spot
Broth dilution inoculum
5.0x10^5/cfu
Standard
0.5% Macfarland Standard
Preparation McFarland
0.5 ml of 1.175% BaCl
99.5 ml of 1% H2SO4
Standard Medium
MHA
pH
7.2-7.4
Agar depth
4 mm or 3-5 mm
Time it takes for drying of inoculum
< 5 minutes
Time it takes for disk diffusion
3-5 minutes (<15 minutes)
Too thick
False Resistant - small ZOI
Too thin
False sensitive - LARGE ZOI
Low pH
“LOW CAE”
Decreased activity:
Clindamycin
Aminoglycoside
Erythromycin
High pH
Decreased acitivity: Tetracycline
High Thymidine
False Resistant - Sulfonamide, Trimethoprin
High Ca, Mg
Decreased aciivity of Aminoglycosides (P. aeroginosa) and Tetracycline (all organisms)
Zone within a zone phenomenon
Swarming Proteous
Inoculum to heavy
Small ZOI (FR)
Inocul
Inoculum to light
Large ZOI (FS)
Inoculated plates incubated in CO2 incubator
CO2 diffuse into the medium, causing the formation of carbonic acid, which will lower th pH of medium
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?
Depth of the media was too thin
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?
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.
In breakpoint antimicrobial drug testing, interpretation of susceptible (S), intermediate (I), and resistant (R) refers to testing antibiotics by using:
Only the specific concentrations necessary to report S, I, or R
