Clinical Use

Question 1

Describe the principles that determine empirical antibiotic use.

Answer 1

1) What is the infectious syndrome that I will be treating (where is the infection located)?
2) What are the common pathogens that might be causing the syndrome?
3) What is the likely antibiotic resistance of the organisms being considered?
4) What is the most appropriate antibiotic to use?
or…
Syndrome/Site
Common Pathogens
Resistance
Appropriate

Question 2

Describe the difference between bactericidal and bacteriostatic antibiotics and how they are defined based on MIC and MBC levels.

Answer 2

STATIC INHIBITS growth of the bug
- MBC/MIC > 4 (it takes a lot more antibiotic to kill the bugs than just to inhibit their growth).

CIDAL KILLS the bug
- MBC/MIC < 4

Question 3

Define MIC and understand the different methodologies used to determine MIC.

Answer 3

The lowest conc of an antimicrobial that will INHIBIT the VISIBLE growth of a microorganism after overnight incubation. Measure via

• tube diultion (visually)
• Kirby-Bauer (measure circle size)

Question 4

Define MBC and understand how an MBC is obtained.

Answer 4

The lowest concentration of the antibiotic that kills 99.9% of the original inoculum in a given time.

• From tube dilution, plate 100uL (which is 10,000 cells, assuming startinc conc of 10^5c/mL)
• If less than 10 colonies, it’s cidal

Question 5

Describe the principles involved in setting antimicrobial breakpoints.

Answer 5

• Pharmacokinetics/dynamics of the antimicrobial
• Pattern of susceptibility of a population of bacteria
• Clinical experience

Question 6

Describe the potential pitfalls in setting AND interpreting antimicrobial breakpoints.

Answer 6

Setting: Lack of clinical data

Interpreting: NOT adjusted for:

• site of infections (can ABX get there?)
• # of organisms (i.e. a closed space abscess won’t respond to ABX unless it is drained first!!)
• conditions in the host (i.e. pH at the site of infection might be low and not amenable to aminoglycoside therapy, as aminoglycosides don’t work in low pH.).
• capability of patient’s host defenses

Question 7

Describe the relationship between maximal serum concentration (Cp) and MIC in order for a bacterium to be potentially considered “susceptible” to a given drug.

Answer 7

Maximal serum concentration (Cp) that exceeds the MIC of the bacteria: Cp > MIC