Antimicrobial susceptibility testing

Question 1

Interpretation: Bacterial resistance is absent or insignificant; appropriate choice for treatment

Answer 1

Susceptible (S)

Question 2

Interpretation: Bacterial susceptibility depends on dosing regimen

Answer 2

Susceptible-Dose Dependent (SDD)

Question 3

Interpretation: Possible use in specific body sites or as a safety margin; not clearly susceptible

Answer 3

Intermediate (I)

Question 4

Interpretation: Microorganism not inhibited; not appropriate for treatment

Answer 4

Resistant (R)

Question 5

Interpretation: Used when only ‘Susceptible’ criteria exist; rare or no resistance cases

Answer 5

Nonsusceptible (NS)

Question 6

Incubation requirement for VRE, VRSA, and MRSA testing

Answer 6

Strict full 24 hours incubation

Question 7

MIC determination in AST

Answer 7

Lowest concentration (TT1-TT5) without growth/clear; MIC = TT6 (with growth/turbid)

Question 8

MBC determination in AST

Answer 8

Broths subcultured on agar; colonies counted to determine 99.9% bacterial reduction

Question 9

Preparation of 0.5 MacFarland standard

Answer 9

99.5 mL of 1% H2SO4 and 0.5 mL of 1.175% BaCl2

Question 10

TT1 and TT6 in AST

Answer 10

TT1 = most concentrated, TT6 = least concentrated

Question 11

Suspension used in Disk Diffusion testing

Answer 11

0.5 MacFarland PURE colony suspension

Question 12

Rotation of swab across MHA plate during inoculation

Answer 12

Three times, turning plate 60° each time

Question 13

Distance between antibiotic disks in Disk Diffusion

Answer 13

24 mm (center-to-center)

Question 14

Maximum number of disks for a 150 mm petri dish

Answer 14

Up to 12 disks

Question 15

Criteria for valid Disk Diffusion test

Answer 15

Pure culture and confluent lawn of growth after incubation

Question 16

Zone reading for Staphylococcus on Methicillin or Enterococci on Vancomycin

Answer 16

Read hazes using transmitted light

Question 17

Proteus swarming during Disk Diffusion

Answer 17

Ignore swarm, measure defined zone of inhibition

Question 18

Zones with sulfonamide or trimethoprim

Answer 18

Ignore hazes, measure zones

Question 19

Light required for reading Disk Diffusion zones

Answer 19

Reflected light with a dark background

Question 20

Reading plates containing blood in Disk Diffusion

Answer 20

Read at the top of the open plate, lid removed

Question 21

CFU/mL required for Disk Diffusion/Kirby Bauer

Answer 21

1.5 x 10^8 or 1-2 x 10^8 CFU/mL

Question 22

Medium used in Kirby Bauer testing

Answer 22

Mueller Hinton agar (add 2% NaCl for MRSA detection)

Question 23

Medium for H. influenzae in Kirby Bauer

Answer 23

Haemophilus test medium

Question 24

Medium for N. gonorrhoeae in Kirby Bauer

Answer 24

GC agar

Question 25

Standard pH and depth for Mueller Hinton agar

Answer 25

pH 7.2–7.4, depth 4 mm

Question 26

Standard incubation conditions for Disk Diffusion

Answer 26

35°C ± 2°C, 16-18 hours, aerobic/ambient air

Question 27

Effect of increased Mg2+ and Ca2+ on P. aeruginosa

Answer 27

False resistance to aminoglycosides and tetracyclines

Question 28

Effect of decreased Mg2+ and Ca2+ on P. aeruginosa

Answer 28

False sensitivity to aminoglycosides and tetracyclines

Question 29

Effect of increased zinc on P. aeruginosa

Answer 29

False resistance to carbapenems

Question 30

Effect of increased thymidine on Enterococci

Answer 30

False resistance to SXT

Question 31

Effect of decreased pH (<7.2) on aminoglycosides, erythromycin, clindamycin

Answer 31

Decreased potency

Question 32

Effect of increased pH (>7.4) on tetracycline

Answer 32

Decreased potency (opposite effect at low pH)

Question 33

Thin media, old cultures, <0.5 MacFarland, delayed incubation, increased drying results in disk diffusion test

Answer 33

False sensitivity (large zone, decreased MIC

Question 34

Thick media, expired antibiotic, >0.5 MacFarland, delayed disk application, increased moisture result in disk diffusion test

Answer 34

False resistance (small zone, increased MIC)

Question 35

Relationship of zone of inhibition to MIC

Answer 35

Inversely proportional

Question 36

Specific concentration separating susceptibility categories

Answer 36

Breakpoint (cut off)

Question 37

Antimicrobial sensitive indicator of drug mechanism

Answer 37

Predictor drug

Question 38

Tracks resistance or susceptibility of organisms to antimicrobials

Answer 38

Antibiogram

Question 39

Trailing in broth dilution (SXT test)

Answer 39

Heavy growth at low antimicrobial concentration, reduced growth in following wells; ignore for SXT, investigate otherwise

Question 40

Skipped wells in broth dilution

Answer 40

Growth at high antimicrobial concentration and no growth at lower concentrations

Question 41

Temperature required for MRSA testing to avoid masking methR

Answer 41

Strict 30–35°C

Question 42

CLSI guidelines for broth and agar dilution methods

Answer 42

CLSI M07

Question 43

CLSI guidelines for disk diffusion testing

Answer 43

CLSI M02 and M100

Question 44

Inoculum size for broth dilution

Answer 44

5 x 10^5 CFU/mL

Question 45

Inoculum size for agar dilution

Answer 45

1 x 10^4 CFU/spot

Question 46

Incubation conditions for broth and agar dilution

Answer 46

35°C, 20-24 hours (16-20 hours for Staphylococcus & Enterobacterales), room air or 5-7% CO2 for capnophiles

Question 47

Media for broth dilution

Answer 47

Cation-adjusted Mueller Hinton broth (CAMHB), Haemophilus test medium for H. influenzae

Question 48

Additive to detect MRSA in AST

Answer 48

2% NaCl

Question 49

Additive for N. meningitidis in AST

Answer 49

2.5-5% lysed horse blood (broth), 5% sheep blood (agar)

Question 50

Purpose of agar dilution method

Answer 50

Tests many organisms against 1 antibiotic, determines MIC for N. gonorrhoeae