It has been generally accepted that the selection of the most effective antibiotic for the treatment of an infection should be based on the result of in vitro sensitivity test. Of a number of methods, the serial dilution method (dilution method) has been considered to be the most reliable, because its result offers a rational information on the therapeutic effect of tested antibiotic in vivo. In addition, dilution method possesses reproducibility and accuracy in the test, when the procedure is carefully standardized and controlled. The disc-plate method (disc method), however, is the most widely used especially in routine laboratories because of its simplicity.
Two methods can be considered to be different in their modes of antibiotic action on the growth of inoculum. In dilution method, antibiotic concentration is constantly maintained in culture medium during incubation period. In disc method, however, the bacterial cells inoculated at any given portion of the surface of an agar plate are influenced by antibiotic, diffusing through from the disc put on a plate, and varying always with its own concentration within a certain time period. Two methods are also different in the manner of reading the results. In dilution method, susceptibility is expressed in terms of minimum inhibitory concentration (MIC), while in disc method, that is determined by measuring the inhibition zone around the disc. Therefore, these facts suggest that the results obtained by both methods may not coincide completely.
On the other hand, method of performing the test, reading and interpreting the results are poorly standardized in disc method, making it difficult to compare results obtained in different laboratories.
Moreover, fundamental analysis of disc method has not yet been completely discussed. In an attempt to make disc method reliable, fundamental study is present in this paper as follows:
1. Study of antibiotic diffusion in disc method, by measuring the actual antibiotic concentration in agar layer, and discussion of the principle of producing the inhibition zone.
2. Observation of various factors affecting the result of disc method.
3. Estimation of the correlation between results of disc and dilution methods.
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