Susceptibility Testing

Worldwide, the agar diffusion test is the generally accepted procedure for determing in vitro sensitivity under routine laboratory conditions. When several antibiotics are tested simultaneously, it is referred to as the antibiotic susceptibility test or antibiogram.

Two methods have been established:

The Disk Diffusion Test and the Epsilon Test (Etest®)*

For the Disk Diffusion Test, a carrier (paper disk) impregnated with a known amount of antibiotic is placed on the surface of a solid medium, which has previously been inoculated with a bacterial suspension of the pathogen to be tested. The antibiotic diffuses from the carrier into the medium, producing a concentration gradient. Bacterial growth in the vicinity of the carrier only occurs when the concentration diffusing from the carrier is no longer sufficient to inhibit bacterial replication, or when the pathogen is resistant to the antibiotic in question. If the concentrations are sufficient to achieve inhibition, a circular region of no bacterial growth develops. This is called the zone of inhibition. In its unmodified form, the agar diffusion test is only suitable for fast-growing aerobes.

The Epsilon Test combines the principle of the agar diffusion test with the determination of the MIC value. Antibiotic carriers in the form of plastic strips contain an antibiotic gradient (from 0.016 - 0.256 µg/ml or from 0.002 - 32 µg/ml).

For pathogens sensitive to the antibiotic, an elliptical zone of inhibition results. The point at which the border of the zone of inhibition crosses the strip is the MIC.

As with the Disk Diffusion Test, several antibiotics can be tested simultaneously.

For Baytril, test disks are available from Bayer. Packsize: 250 susceptibility disks (5 cartridges with 50 disks each) Etest®* for Enrofloxacin is available at AB BIODISK unter article number 5100 2898.
www.abbiodisk.com

*Etest® is a registered trademark of AB BIODISK and patented in all major markets.