Evaluation of in vitro sensitivity testing methods of detecting sulphamethoxazole trimethoprim synergy. Correlation with mouse protection assay.
To evaluate the applicability of sensitivity tests in the detection of synergy between sulphamethoxazole (SMX) and trimethoprim (TMP) hospital strains of Escherichia coli were tsted by the disc sensitivity and the agar dilution tests using Isosensitest agar; sensitivity to SMX, TMP and 20 parts sulphamethoxazole plus 1 part trimethoprim (SXT) was determined. Ten of 19 strains of E. coli showed larger zones of inhibition with the SXT discs than with either SMX or TMP discs. Yet all 19 E. coli strains showed appreciably lower MIC for SXT than either SMX or TMP in the agar dilution test. It is concluded that disc sensitivity tests do not detect synergy between SMX and TMP. Demonstration of a lower MIC with SXT than either SMX or TMP in the agar dilution test is a better indication of synergy. This conclusion is confirmed in the mouse protection tests.
D000818:Animals; D001419:Bacteria / Q000187:drug effects*; D004357:Drug Synergism; D004927:Escherichia coli Infections / Q000517:prevention & control; D051379:Mice; D008826:Microbial Sensitivity Tests / Q000379:methods; D013420:Sulfamethoxazole / Q000494:pharmacology* / Q000627:therapeutic use; D014295:Trimethoprim / Q000494:pharmacology* / Q000627:therapeutic use
D014295:Trimethoprim; D013420:Sulfamethoxazole