Antimicrobial Drugs


32. Describe the Kirby-Bauer method of antibiotic susceptibility testing.

33. Describe how the results of the Kirby-Bauer procedure are interpreted.

34. Describe the use of broth dilution tests.

Tests To Guide Chemotherapy

These tests are used to determine which chemotherapeutic agent is most likely to combat a specific pathogen.

These tests are used when susceptibility cannot be predicted or when drug resistance arises.

The Diffusion Methods

In this test, also known as the Kirby-Bauer test, a bacterial culture is inoculated on an agar medium, and filter paper disks impregnated with chemotherapeutic agents are overlaid on the culture.

After incubation, the absence of microbial growth around a disk is called a zone of inhibition.

The diameter of the zone of inhibition, when compared with a standardized reference table, is used to determine whether the organism is sensitive, intermediate, or resistant to the drug.


Disk Diffusion


MIC is the lowest concentration of drug capable of preventing microbial growth; MIC can be estimated using the E test.

A plastic coated strip contains a gradient of antibiotic concentrations and the minimal inhibitory concentration is read from a scale printed on the strip.


E test

Broth DilutionTests

In a broth dilution test, the microorganism is grown in a liquid media containing different concentrations of a chemotherapeutic agent.

The lowest concentration of a chemotherapeutic agent that kills bacteria is called the minimum bactericidal concentration (MBC).