The principle of the disc diffusion method is that when a filter disc is impregnated with a chemical that is placed on the agar, the chemical in the agar diffuses around the disc. The solubility of the chemical and its molecular size determines the size of the area if chemical infiltration around the disc. When an organism is placed on the agar around the disc, if it is susceptible to the chemical then there would be no growth around the area where the chemical is diffused and this area is called as the "zone o the inhibition".
This inhibition can be measured and compared with a standard control strain and normal tables, Stokes and Kirby-Bauer method respectively. The factors that affect in this method are the concentration of the bacterial inoculums, the depth and types of agar, the incubation conditions and the time of incubation. All this factors should be always taken into account while performing the test. An alternative test that can be performed is the determination of the amount of antibiotic required to either inhibit the growth of organism or to kill the organism which is done by incubating a fixed concentration of the organism in increasing concentrations of antibiotics and checking for growth after 24h of incubation.
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