Determination of Bacterial Resistance to Trimethoprim/Sulphamethoxazole Using the Single Disc Diffusion Method

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Abstract
The validity of the disc diffusion method with a combined disc in determining bacterial resistance to trimethoprim/sulphamethoxazole (TMP/SMZ) has been investigated. With a 25-μg disc containing 1 part of TMP and 19 parts of SMZ, a strong correlation between ‘minimal inhibitory concentration’ (MIC) values (determined with the agar dilution method according to the ‘International Collaborative Study’ (ICS)) and zone diameters (determined with the standardized disc procedure of the United States Food and Drug Administration (FDA)) was observed. A strong correlation was equally observed using Mueller-Hinton agar (BBL) plus 5% citrated human blood and Wellcotest agar. A tentative zone diameter of 16 mm or more is recommended as the borderline in the differentiation of susceptible bacteria from resistant strains. The formation of an intermediate group is not proposed. The categorization of organisms was developed from the relationship of MIC values to mean steady state concentrations of the drug in the blood, and from the distribution of susceptibilities among bacteria.

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