BSCI 424 — PATHOGENIC MICROBIOLOGY — Fall 2000

In Vitro Antibiotic Susceptibility Testing

Performed to determine the susceptibility of the organism isolated from the diseased host; Generate antibiogram(s)

Broth Dilution MIC test

Agar Dilution MIC test

Broth/Agar MBC test

Agar Diffusion (Kirby-Bauer Disk) Test: Measure diameter of zone of inhibition; Read as susceptible, intermediate, or resistant

Factors influencing zones of inhibition on agar

Concentration of bacteria spread onto agar plate
Pathogen susceptibility
Antibiotic diffusion effects
Agar depth
Growth rate
Temperature
Nutrient availability
Drug antagonists

Factors influencing diffusion of antibiotic

Concentration of antibiotic: Kirby-Bauer disks, E-tests have standardized concentrations
Molecular weight of antibiotic
Water solubility of antibiotic
pH and ionization
Binding to agar

Modern Commercial Kits (e.g., E-test strips (up to six) on an agar plate and can be read out as MIC)

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