Antimicrobial Susceptibility Testing

Question 1

Disk diffusion testing

Answer 1

as the distance from the disk increases, the concentration of antimicrobial decreases creating a gradient of drug concentrations surrounding each disk
diameter of the zone of inhibition influenced by the rate of diffusion of the agent though the agar which may vary depending on the size and solubility of the drug

Question 2

Broth dilution methods

Answer 2

used to determine the MIC of the antimicrobial required to inhibit or kill a bacterium

Question 3

MIC

Answer 3

antimicrobials tested as twofold serial dilutions

lowest concentration that inhibits visible growth

Question 4

Gradient diffusion method

Answer 4

E test

Question 5

E test

Answer 5

method for quantitative susceptibility testing that incorporates a preformed gradient applied to one side of a plastic strip to provide drug diffusion into the agar
following incubation MIC is read directly from a scale on top at the point where the ellipse of bacterial growth inhibition intercepts the strip

Question 6

Detection of methicillin (R) Staph aureus

Answer 6

utilizing disk diffusion testing the antimicrobial cefoxitin used as a surrogate marker for mecA-mediated oxacillin (R)

Question 7

Clindamycin resistance

Answer 7

due to methylation of the ribosome which prevents erythromycin and clindamycin from binding to 50S ribosome subunit
methylation coded for by erm gene

Question 8

Detection of inducible clindamycin (R)

Answer 8

D test

Question 9

D test

Answer 9

utilizing disk diffusion testing erythromycin used to induce clindamycin (R)
flattening of the zone of inhibition adjacent to erythromycin disk = inducible clindamycin (R)

Question 10

Confirmatory test for extended spectrum B lactamase (ESBL)

Answer 10

utilizing disk diffusion testing Ceftazidime and Cefotaxime tested with and without the B lactamase inhibitor Clavulanic acid
>5mm increase in zone diameter for either antimicrobial in combo with clavulanic acid vs zone diameter of antimicrobial tested alone = (+)

Question 11

Confirmatory testing for suspected carbapenemase production (CRE) in gram neg bacteria

Answer 11

Modified Hodge Test

Question 12

Modified Hodge test

Answer 12

utilizing disk diffusion testing Ertapenem (carbapenem) and E coli ATCC strain 25922 used to detect production of carbapenemase in test bacterium
enhanced growth = (+)