Antimicrobial Susceptibility Testing

Question 1

What are the 2 main purposes of susceptibility testing ?

Answer 1

1. treatment guide

2. epidemiological tool (ie tracking resistance)

Question 2

What are the 7 components in susceptibility testing

Answer 2

1. ID of organism
2. ID of infection site
3. Selection of antibiotics
4. Selection of appropriate test method
5. Interpretation
6. Selective reporting
7. Quality control

Question 3

What 4 things do you want to look for when you are determining the organism?

Answer 3

1. Predictably susceptible?
2. Intrinsic resistance
3. Inducible resistance
4. Hetero-resistance phenotypes

Question 4

What are ß-haemolytic strep never resistant to ?

Answer 4

ß-lactams

Question 5

What drug are gram negative bacteria intrinsically resistant to?

Answer 5

Vancomycin

Question 6

What drug are the enterococci intrinsically resistant to?

Answer 6

Cephalosporins

Question 7

What considerations need to be taken in relation to site of infection?

Answer 7

if in the brain, need a drug that will cross the blood/brain barrier

If you have a UTI you need a drug that will be excreted renally vs hepatically

If you have an RTI you cant use Daptomycin

Question 8

What factors ensure the quality control of the testing?

Answer 8

Reference strains (ATCC)
- MIC ranges / zone diameters
Media - pH, depth, thymidine content, divalent cation content
Antibiotics - shelf life, potency
Incubation temp, time, atmosphere, inoculum

Question 9

What does the nitrocephin disk test for?

Answer 9

Tests for ß-lactamase production

Question 10

What is MIC?

Answer 10

Minimum Inhibition Concentration (MIC) = The lowest concentration of antimicrobial agent that inhibits bacterial growth/ multiplication

Question 11

What is another name for the disc-diffusion method?

Answer 11

The Kirby-Bauer test

Question 12

How does the Kirby-Bauer/Disc-diffusion test work?

Answer 12

Antibiotic-impregnated filter disc
Susceptibility test against more than one antibiotic by measuring size of “inhibition zone ”
- gives you a qualitative result that is comparable to quantitative results from MIC tests

Question 13

What 10 factors can affect the size of the zone of inhibition

Answer 13

1. Inoculum density
2. Timing of disc application
3. Temperature of incubation
4. Incubation time
5. Depth of agar
6. Spacing of discs
7. Potency of antibiotic discs
8. Composition of media
9. pH of media
10. Reading of the zones (subjective errors)

Question 14

How does the inoculum density affect the ZoI size?

Answer 14

lighter inoculum density will give you larger zones and vice versa

Question 15

How does the timing of disc application affect the ZoI size?

Answer 15

If after application of disc, the plate is kept for longer time at room temperature, small zones may form

Question 16

How does the temperature affect the ZoI size?

Answer 16

Larger zones are seen with temperatures < 35ºC

Question 17

what is the ideal incubation time?

Answer 17

16-20 hours

Question 18

What is the ideal depth of agar? how does deviation from this affect the ZoI ?

Answer 18

4 mm

Thin media yield excessively large inhibition zones and vice versa

Question 19

What zones of inhibition (for certain drug types) are larger in acidic pH ?

Answer 19

Tetracycline, novobiocin, methicillin zones are larger

Question 20

What zones of inhibition (for certain drug types) are larger in basic pH ?

Answer 20

Aminoglycosides, erythromycin

Question 21

How does double diffusion disk test work for the macrolide resistance phenotypes?

Answer 21

You have a clindamycin (lincosamide) and a macrolide disk near each other on a dish

1. inducible MLS phenotype will show the “D test” - blunting of the ZoI
2. constitutive MLS phenotype will show no ZoI around either disk
3. M phenotype. Big ZoI around the clindamycin, small to none around the macrolide

Question 22

How do you read the MIC from an E test?

Answer 22

The point (read from scale) where the zone of inhibition intersect the strip

Question 23

What is the MBC?

Answer 23

Minimum Bactericidal Concentration (MBC) or Minimum Lethal Concentration (MLC) = The lowest concentration of antimicrobial agent that allows less than 0.1% of the original inoculum to survive

Question 24

How do you determine MIC and MBC from broth dilution?

Answer 24

for MIC you look for turbidity so the first dilution at which you can see any growth with the naked eye

For MBC you subculture all the non-turbid tubes and see at which concentration you have <0.1% growth

Question 25

What’s the procedure for agar dilution?

Answer 25

Using a replicating inoculator device called “A Steers-Foltz replicator” deliver 0.001 ml of bacterial inoculum into an agar with known concentration of antibiotic

Put a bunch of different bacteria onto a single plate

Question 26

What is the name for the automated system of susceptibility testing ?

Answer 26

Vitek