Antimicrobial Susceptibility

Question 1

when should you NOT perform susceptibility testing? (3)

Answer 1

• probable pathogen not identified
• systemic antimicrobial therapy will not be used
• significant pathogen has a predicatble susceptibility pattern

Question 2

Which 9 organisms are predictably susceptible?

Answer 2

1. beta hemolytic streps
2. clostridium
3. erysipelothrix rhusiopathiae
4. listeria monocytogenes
5. bacillus anthracis
6. actinomyces spp
7. haemophilus spp.
8. pastuerella spp
9. fusobacterium spp

Question 3

T/F: if a bacterial pathogen is predictably susceptible, treatment will be easy

Answer 3

false, it just means that in vitro resistance to first line therapies for that organism has not been reported.

exs of predictably susceptible athogens that are NOT easy to tx are bacillus anthracis, clostridium, and actinomyces bovis

Question 4

when is susceptibility indicated?

Answer 4

1. sample collected appropriately
2. sample transported appropriately to lab
3. clinical evidence of inflammation
4. significant growth isolated and not consistent with contamination.
5. you want to use systemic antibiotics

Question 5

what happens when you run susceptibiliy on an organism with questionable clinical signfiicance?

Answer 5

you may get skewed results. It may say that the organism is resistant to antimicrobials that it is not.

Question 6

_____________ means bacterial growth is inhibited by a concentration of antibacterial associated with high likelihood of therapeutic success.

Answer 6

susceptible

Question 7

__________ means bacterial growth is inhibited by a concentration of an antibacterial associated with an uncertain therapeutic effect.

Answer 7

intermediate

Question 8

_________ means bacterial growth is inhibited by a concentration of antibacterial associated with high likelihood or therapeutic failure

Answer 8

resistant

Question 9

T/F: susceptibility category intermediate drugs may have efficacy in body sites where the drug is physiologically concentrated.

Answer 9

true

Question 10

Describe the 90-60 rule

Answer 10

if a patient has a monobacterial infection and is immunocompetent, then…
90% pts will have a positive therapeutic response using ‘susceptible’ drugs
60% pts will have a will have a positive therapeutic response using ‘resistant’ drugs

This is due to the help of the immune system and proves that in most cases, even if we do nothing at all, the patient will have a positive outcome. Our goal is to close the 90-60 gap.

Question 11

If you started empirical antibiotic treatment on a patient with a UTI and their culture results inform you that the current antibiotic is “intermediate”, what do you do?

Answer 11

Call the client and get an update on the patient’s condition
If they are improving, you can finish the course of current antibiotics.
If they are not doing well, you can switch to a drug listed as “susceptible”

Question 12

In what scenario would using “intermediate” drugs be MOST appropriate and therefore unecessary to switch to a “susceptible” drug?

Answer 12

• young, healthy animal
• uncomplicated infection
• infection is clinically responding well to therapy

Question 13

does it matter if a lab uses microbroth dilution vs K-B disk diffusion?

Answer 13

no

Question 14

________ is the minimum concentration of drug required to inhibit the organism’s growth and can only be dervied from broth micro dilution or gradient strip testing.

Answer 14

MIC

Question 15

why is it important that you indicate the site from which the sample was collected when submitting c/s?

Answer 15

The amount of drug achievable at different sites of the body is variable. The lab needs to know the site so that they can use the correct breakpoint for determining susceptible, intermediate, or resistance.

Question 16

________ is the maximum MIC that predicts sucessful therapy and tells you if an organism is susceptible, intermediate, or resistant.

Answer 16

breakpoint

Question 17

what 3 things do breakpoints take into account?

Answer 17

1. the normal range of MIC values of wild type bacteria
2. PK/PD properties of the drug in the species of interest
3. the site of infection

Question 18

T/F: there is a breakpoint for each source species, drug, organism, and site combination

Answer 18

true

Question 19

T/F: picking a drug off the susceptibility results list requires picking the drug with the lowest MIC value

Answer 19

false
this is inaccurate because each source species, drug, organism, and site has its own breakpoint, so choosing a drug based on its MIC value alone is not significant. A MIC for doxycycline may be 0.12, but perhaps thats right at the breakpoint, whereas the MIC for cephalexin may be 0.5 and the breakpoint is 2. In general, a susceptible drug is a susceptible drug.

You must consider other things like side effects, penetration to site of infection, cost, dosing convenience, owner compliance, etc.

Question 20

what do we do when there is no breakpoint?

Answer 20

use breakpoints from other species or from similar organisms.

Question 21

T/F: breakpoints stay consistent over time

Answer 21

false

Question 22

T/F: MIC breakpoints are based on topical and systemic concentrations of antimicrobials

Answer 22

false – only systemic concentrations.

Topical antimicrobials have much higher concentrations than can be achieved through oral or parenteral dosing.

Question 23

why is the susceptibility list of drugs so short for organisms like pseudomonas aeruginosa and enterococcus faecium?

Answer 23

high level of intrinsic resistance! labs do not want you to mistake it for some highly resistant pathogen.

P. aeruginosa is intrinsically resistant to beta-lactams, most cephalosporins, tetracyclines, chloramphenicol, trimethoprim-sulfa, and easily develops resistance to fluoroquinolones.
So, the list of drugs is always shorter.

E. faecium is intrinsically resistant to aminoglycosides, cephalosporins, clindamycin, trimethoprim-sulfa