Clinical Uses of Antibiotics Flashcards
What are some potential pitfalls associated with specimen collection, Gram-stains, and cultures?
Samples of organisms must be obtained BEFORE starting antibiotic therapy. Samples collected after may be unrepresentative. Tissues or fluids are most appropriate samples, “swabs” tend to result in poor recovery of infecting organisms (esp. anaerobic bac). Superficially obtained specimens may not contain the infecting organism, and instead may show colonizing bac leading to inappropriate treatment. Cultures obtained from catheters or drains are highly suspect as any existing catheter/drain that has an external opening will become colonized with bac different from the infecting agent.
What are the principles of empiric antibiotic use?
Antibacterial treatment must frequently be started before the specific causative agent is known. One must apply knowledge of the organisms most likely to cause the infection in a given clinical setting. Life threatening infections should be treated empirically immediately, followed by spec treatment when the cause is known. Outpatient treatments may be conducted with no culture at all (otitis media). Antibiograms are published annually to show the most likely causes of infections in local regions throughout the year and are useful in guiding therapy choices.
What is the broth dilution method of determining antibiotic susceptibility?
Broth dilution - Serial dilutions of an antibiotic are made and then inoculated with a standardized number of organisms for a prescribed time. The vials are visually examined and the vial with the lowest concentration of antibiotic that prevents visible bac growth is found. This concentration is deemed the Minimum Inhibitory Concentration (MIC).
What are the Disk Diffusion (Kirby-Bauer) and E-test methods of determining antibiotic susceptibility?
Disk Diffusion - a predetermined concentration of bacteria is spread on a large agar plate and disks containing known concentrations of antibacterial drugs are placed on the agar. After incubation, the diameter of clearing around the disk is measured. E-Test - A variant of the disk diffusion test wherein a strip containing a concentration gradient of a drug is placed on the agar. After incubation, an elliptical pattern is visible on the plate and the point where the ellipse meets the strip determines the MIC.
How is MBC calculated and how is it used with MIC to classify drugs as bacteriostatic or bactericidal?
The Minimum Bactericidal Concentration (MBC) is the lowest concentration of drug that kills 99.9% of bacteria in a given time. Antibacterial drugs that have MBC equal to or barely above their MIC are termed bactericidal, those with MBC concentrations many fold higher than the MIC are termed bacteriostatic.
What are the antibacterial susceptibility categories?
Antibacterial drugs are categorized as susceptible, intermediate (or Susceptible Dose Dependent), or resistant based on the determined MIC from the broth or E-test, or the diameter of clearance from the disk diffusion test.
What are the four caveats of the susceptibility test results?
1) They are NOT adjusted for the site of the infection (i.e. ability of antibiotics to penetrate into the site) 2) NOT adjusted for the number of organisms in an infection (i.e. closed abscesses must be drained first!) 3) NOT adjusted to conditions within the host (i.e. low pH in the site of the infection may interrupt drug action) 4) NOT adjusted to reflect the patient’s host defenses. These are all scenarios that may affect the clinical outcome of patients independent of the bacteria’s susceptibility.
What is the general guideline for a bacteria to actually be considered susceptible to an antibiotic?
The categories of susceptible, intermediate, and resistant have boundaries defined by the Clinical and Laboratory Standards Institute (not shown on this slide). However, the general guideline for a bacteria to be considered susceptible to an antibiotic is that at normal doses, the antibiotic should have a maximal serum concentration that exceeds the MIC of the bacteria. i.e. Cp > MIC
