Toxoplasma gondii: characterization of a mutant resistant to sulfonamides

Sulfadiazine was a potent inhibitor of the in vitro growth of Toxoplasma gondii, although it had little effect during the first 24 hr of treatment. A mutant parasite (R-SulR-5) with a 300-fold increase in sulfadiazine resistance was selected by a combination of chemical mutagenesis and growth in gradually increased sulfadiazine concentrations. This mutant was completely cross-resistant to several other sulfonamides and to dapsone. The same concentration of p-aminobenzoic acid reversed the sulfadiazine inhibition of both mutant and wild-type parasites even though much higher concentrations of sulfadiazine were used to inhibit the mutant. Dihydropteroate synthase, a sulfonamide-sensitive enzyme in the pathway leading to dihydrofolic acid, had similar activities in wild-type and R-SulR-5 parasites. However, the mutant enzyme was 40-fold more resistant to sulfadiazine and had higher apparent Kms for both substrates, p-aminobenzoic acid and dihydropteridine pyrophosphate. The mutant was slightly less active than the wild type in the uptake of sulfadiazine.