Regression-line Analysis of Trimethoprim-Sulfamethoxazole Activity againstNeisseria gonorrhoeae

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Abstract
A standardized disk diffusion test was developed and used to test the susceptibility of 102 strains of Neisseria gonorrhoeae to combinations of trimethoprim and sulfamethoxazole (TMP/SMX) by relating zone diameters of inhibition to minimal inhibitory concentrations (MIC’s). MIC’s for TMP/SMX in ratios of 1:20 ranged from 0.08/1.52 to 2.5/47.5 µg/ml and zones of inhibition ranged from 34 to 10 mm. The coefficient of correlation (r) was -0.75. For comparison, a regression line was similarly calculated for ampicillin. MIC’s ranged from 0.02 to 0.32 µg/ml and zones of inhibition ranged from 50 to 31 mm; r was -0 . 7 1. With establishment of MIC breakpoints to define the categories, susceptible, intermediate, and resistant, the disk duffusion test would be as reliable for estimating susceptibility of gonococci to TMP/SMX as for estimating susceptibility to ampicillin.