Zone of Inhibition Test : Antimicrobial Activity Screen (With Photo)

Zone of Inhibition Test - Kirby-Bauer Test

Welcome to Antimicrobial Test Laboratories' Zone of Inhibition Test Resource Page. The assay pictured here is also sometimes called a Kirby-Bauer Test and is used regularly by clinical laboratories under CLSI guidelines to determine antbiotic resistance of bacterial isolates.

Photograph: A zone of Inhibition is evident around the oxacillin disk for S. aureus, left, but not for Methicillin-resistant S. aureus (MRSA), right.

Below, you will find a summary of the Kirby-Bauer Zone of Inhibition test method, along with some of its strengths and weaknesses. The Kirby-Bauer Zone of Inhibition test method is designed to qualitatively test the ability of antimicrobial agents to inhibit the growth of microorganisms over an 18-24 hour period of contact.

Summary of the Zone of Inhibition Test:

A microbial suspension is spread by a sterile swab, evenly, over the face of a sterile agar plate.
The antimicrobial agent is applied to the center of the agar plate (in a fashion such that the antimicrobial doesn't spread out from the center) and incubated.
If substantial antimicrobial activity is present, then a zone of inhibition appears around the test product. The zone of inhibition is simply the area on the agar plate that remains free from microbial growth.
The size of the zone of inhibition is usually related to the level of antimicrobial activity present in the sample or product - a larger zone of inhibition usually means that the antimicrobial is more potent.

Strengths of Zone of Inhibition Testing:

Zone of inhibition testing is fast and inexpensive relative to other laboratory tests for antimicrobial activity.
Zone of inhibition testing is especially well suited for determining (albeit qualitatively) the ability of water-soluble antimicrobials to inhibit the growth of microorganisms.
A number of samples can be screened for antimicrobial properties quickly using this test method.
A variety of antimicrobial product types can be tested using this method. Liquids, coated antimicrobial surfaces, and antimicrobial-impregnated solid products can all be tested for their ability to produce a "zone of inhibition."

Weaknesses of Zone of Inhibition Testing:

Zone of Inhibition tests do not necessarily indicate that microorganisms have been killed by an antimicrobial product - just that they have been prevented from growing.
Microbial growth agars themselves may interfere with the function of some antimicrobial agents. bullet The method cannot be used to test the activity of antimicrobial agents against viruses, since viruses don't "grow" on agar plates like bacteria (viruses don't replicate outside of their host organisms).
The method has some natural variability, and zones of microbial inhibition do not always have clear or regular boundaries. bullet The method is not classically quantitative (though sometimes the diameter of the zones of inhibition are measured and recorded).

Zone of Inhibition Testing is a fast, qualitative means to measure the ability of an antimicrobial agent to inhibit the growth of microorganisms.

Antimicrobial Test Laboratories was built around antimicrobial innovation and antimicrobial research and development. If your company is interested in screening chemicals, products, or antimicrobial treatments for their ability to inhibit the growth of microorganisms, the zone of inhibition test may be a great place to start.

For more information about Zone of Inhibition testing, Contact the Lab Today!