Problems in the checkerboard method using serial twofold dilutions.

Toyokawa Masahiro, Asari Seishi, Horikawa Masayuki, Tsukamoto Hisako, Tahara Kazuko, Sunada Atsuko, Amino Nobuyuki

doi:10.11250/chemotherapy1953.42.841

We measured in vitro the effect of combined therapy against methicillin-resistant Staphylococcus aureus (MRSA) using the modified checkerboard method with serial 1.25-fold dilutions (1.25-fold method) and a conventional method using serial twofold dilutions (twofold method) to compare the results. Vancomycin and minocyclin, which have a low MIC value, and cefotiam, which has a high MIC value, were used. In the determination of MIC value, discrepancies were found between the twofold and 1.25-fold methods even in the same strain. Discrepancies between these two methods were also observed in the combination effect in some cases. In the twofold method, decreased intensity of the MIC value in combination could be detected only in the form of (1/2) ⁿ. Therefore, in this method results with a 1/2 decrease were outwardly obtained even with a decrease of less than 1/2. On the other hand, in the 1.25-fold method it could be detected in the form of (4/5) ⁿ, and no discrepancies were found between the results of this method and the expected decrease. Thus more exact FIC indexes and MIC values in combination were obtained. In addition, the present method could detect weak antagonism that could not be detected by the twofold method. We concluded that 1.25-fold serial dilution was better than twofold dilution in the assesment of combinations of an antibiotic with a low MIC value and one with a high MIC value.

Problems in the checkerboard method using serial twofold dilutions.

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