In-vitro bactericidal synergism of rifampicin and trimethoprim and implications for treatment of carriers of Haemophilus influenzae

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Abstract
Rifampicin (RIF) and trimethoprim (TMP), when used separately for prophylaxis, do not always eradicate Haemophilus influenzae from the upper respiratory tract. We studied the in-vitro activity of various combinations of these two drugs on these bacteria. Twenty-nine strains of H. influenzae were tested for susceptibility to RIF and TMP, separately and in combination. Two H. influenzae type b strains were also selected for kill curve studies (TMP was bactericidal for one strain and not for the other). The mode minimum inhibitory concentrations (MICs) for RIF and TMP were 1–0 and 0–12 mg/1, respectively. TMP was not bactericidal for 84% of the strains. The MIC and the minimum bactericidal concentration (MBC) for RIF were the same. Chequerboard MBC data showed synergism, but MIC data did not. By MBC, RIF and TMP were synergistic at a 1:1 ratio, with 97% of strains killed at 0–l2/0–12 mg/1.