Susceptibility Testing

Question 1

What are the limitations of antimicrobial susceptibility testing?

Question 2

MIC vs MBC

Answer 2

MIC: Minimum Inhibitory Concentration
- lowest [antimicrobial] required to inhibit microbe growth

MBC: Minimum Bactericidal Concentration
- lowest [antimicrobial] required to kill microbe

Question 3

Bacteriostatic vs bactericidal drugs’s MBC:MIC ratio

Answer 3

• Bacteriostatic = MBC:MIC > 4
• Bactericidal = MBC:MIC ≤ 4

Question 4

What test is this? What is the MIC?

Answer 4

E-test strip – diffusion/gradient method
- MIC = where bacteria growth meets strip
- Strip has an increasing concentration of antibiotic; agar plate has standardized concentration of bacteria added

Question 5

Why should bacteriostatic drugs never be used as sole drug in immunocompromised patients?

Answer 5

B/c their effectiveness is dependent on the host’s immunity response
- bacteriostatic drugs delay bacteria growth until the host’s immunity can kill them

Question 6

Breakpoints

Answer 6

Maximum MIC that predicts successful therapy -> tell you if organism is S, I, or R
- ideal breakpoints combine patient species + drug properties + bacterial species + site of infection

based on SYSTEMIC CONCENTRATIONS

Question 7

Concentration associated with uncertain therapeutic effect

Answer 7

Intermediate
- may have effect in body sites where drugs are physiologically concentrated

Question 7

Concentration is associated with high likelihood of therapeutic success

Answer 7

Susceptible
- bacteria is inhibited by achieveable concentrations of antimicrobial when recommended dosage is used

Question 8

Concentration associated with high likelihood of therapeutic failure

Answer 8

Resistant
- bacteria NOT inhibited by achieveable concentrations of antimicrobial /are likely to have specific resistance mechanisms that will limit success

Question 9

Who creates breakpoints in the U.S.?

Answer 9

CLSI (clinical and laboratory standards institute) – based on published PK/PD data, noraml range of MIC values of particular bacterial species, sometimes site of infection

Question 10

Multidrug resistance

Answer 10

phenotypic antimicrobial resistance to ≥ 3 antimicrobial drug classes
- phenotypic determiend by AST and breakpoints

e.g., pseudomonas is prone to MDR

Question 11

You want to use topical abx. Is AST indicated?

Answer 11

No- AST can only give info based on systemic concentrations
- topical abx reach much higher concentrations than oral or parenteral dosing

Question 12

When is susceptibility testing NOT needed? (4)

Answer 12

1. probable pathogen has not been identified
2. significant pathogen has a predictable sus pattern
3. if fastidious organisms sus, they do not yiel sus results
4. topical therapy is indicated

Fastidious bacteria = difficult to grow in the laboratory because they have complex or restricted nutritional and/or environmental requirements (heliobacter, campylobacter)

Question 13

Young, healthy dog with 1st time UTI by E. coli is being empirically treated with enrofloxacin. Aerobic C&S results return and enro is listed as “intermediate” - should you switch your drug?

Answer 13

No- enro concentrates well in the urine and even though it is intermediate, the patient is young/healthy with an uncomplicated infection. Intermediate = minimum of [antimicrobial] needed to inhibit bacterial growth associated with an uncertain therapeutic effect – just b/c intermediate, doesn’t mean it won’t elicit clinical response

Question 14

Why should rifampin NEVER be used as a SOLE antimicrobial when treating MRSP infection?

Answer 14

MRSP can gain resistance to rifampin within 48h of use when sole antimicrobial!
- often combined with macrolides or fluoroquinolones

Question 15

Cat with cholangitis has culture growing both enterococcus (gram +, cocci, facultative anaerobe) and **E. coli **(gram-, facultative anaerobe, rod, coliform). What abx would you treat each?

Answer 15

1. Enterococcus = ampicillin or clavamox (amoxicillin/clavulanic acid)
2. E. coli = enrofloxacin

Clavulanic acid is a beta-lactamase inhibitor that is frequently combined with Amoxicillin or Ticarcillin to fight antibiotic resistance by preventing their degradation by beta-lactamase enzymes, broadening their spectrum of susceptible bacterial infections.

Question 16

Are there any antimicrobials that can eradicate biofilms?

Answer 16

No

Biofilm reduce or fully prevent antibiotic effectiveness and further drive resistance. Allow for the survival of organisms within the biofilm in the presence of high concentrations of antibiotics, a phenomenon known as recalcitrance