The interactions between rifampicin and trimethoprim: an in vitro study

Abstract

An in vitro study of the interactions between rifampicin (RIF) and trimethoprim (TMP) against 100 urinary pathogens of various species and sensitivity patterns has been conducted using ‘chequer board’ titrations on solid media. Strong synergism was shown with 1 Escherichia coli and 1 Staphylococcus albus strain. Moderate synergism was demonstrated with 9 strains of Streptococcus faecalis, 4 strains of Staph. albus and 1 strain of Proteus mirabilis. No synergy could be demonstrated against the strains of Klebsiella spp. tested. Of the 16 strains showing synergistic action of RIF and TMP, 2 strains of Strep, faecalis were doubly-sensitive, but of the others 9 (5 Strep, faecalis, 3 Staph. albus and 1 Proteus mirabilis) were resistant to TMP but sensitive to RIF, and 5 (2 Strep, faecalis, 2 Staph. albus and 1 E. coli) were doubly resistant. The synergism seen was likely to be clinically significant with 11 of the 16 strains. In this study antagonism was not seen with any strain.