KIRBY-BAUER

Disk diffusion susceptibility test

Kirby-Bauer test is widely used to determine the sensitivity or resistance of bacteria to various antimicrobial compounds, and it uses the Mueller Hinton agar.

Mueller-Hinton agar is a non-selective, non-differential medium capable of growing a wide range of non-fastidious organisms. It is considered a ‘loose’ agar, which helps to mediate the rate of diffusion of the antimicrobial more effectively than any other types of media.

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The Kirby-Bauer procedure

Mueller Hinton agar is aseptically inoculated with bacteria via the spread method.

1. Sample Preparation & Incubation

A range of discs impregnated with an antimicrobial are placed on the surface for incubation at 35°C for 24 hours.

2. zone of inhibition Monitoring

After incubation, the plates are observed for a ‘zone of inhibition’; a circular area relating to the level of antimicrobial activity upon the bacteria. Any zone of inhibition present indicates a degree of sensitivity to the antimicrobial; the larger the zone, the more potent the antimicrobial. On the other hand, no zone indicates total bacterial resistance.

3. Measuring Susceptibility

The susceptibility of the bacteria can then be measured by taking the diameter of the inhibition zone from the centre of the disc and matching the number to a standard of values, depending on the antimicrobial and concentration.

Things to note 

Applications

Whilst this method is primarily used for antibiotic susceptibility, it can be used to test other antimicrobials, such as the effectiveness of detergents and hand sanitisers.

Testing using the Kirby-Bauer method can for example determine whether a microorganism, such as MRSA, is resistant to a specific antimicrobial. This could be useful to support research in finding a suitable detergent for laboratories to help make informed decisions on the applicability of a cleaning agent.

In recent years, there has been a lot of interest in antimicrobials, such as research into the antimicrobial activity of silver nanoparticles with and without the combination of antibiotics.

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