Background Although there are several drugs and drug combinations for the treatment of Pneumocystis carinii (P. carinii) pne umonia, all drugs have the toxicity as well as low efficacy. Iron chelators have been proposed as a source of new drugs for combating these infections. We hypothesized that iron chelators would suppress the growth of P. carinii by deprivation of the nutritional iron required for growth. In this study, a short-term axenic culture system of P. carinii was established. Daphnetin (7,8-dihydroxycoumarin), a known iron chelator, was demonstrated to exhibit in vitro activity against P. carinii in this system. Methods P. carinii organisms were obtained from the lungs of immunosuppressed rats. The culture system consisted of Iscove Dulbecco Eagle’s Minimum Essential Medium (IMDM), supplemented with S-adenosyl-L-methionine, N-acetylglucosamine, putrescine, L-cysteine, L-glutamine, 2-mercaptoethanol, and fetal bovine serum, and was maintained at 37℃, in 5% CO 2, 95% O 2, at the optimal pH of 8.0. The culture system was used to assess the effect of daphnetin on the proliferation of P. carinii organisms. The ultrastructures of the treated organisms were observed by transmission electron microscopy.Results The number of cysts and trophozoites increased 8- to 9-fold and 11- to 12-fold, respectively, after 10 days of culture. Daphnetin was found to suppress the growth of P. carinii in a dose-dependent manner at concentrations between 1 μmol/L and 20 μmol/L. The inhibitory activity was suppressed by the chelation of daphnetin with ferrous sulfate in a 2∶1 molar ratio, but it was not suppressed by mixing the culture medium with magnesium sulfate. Reduction of P. carinii numbers after treatment with daphnetin correlated with morphological changes in the organisms, as determined by transmission electron microscopy.Conclusions Daphnetin can suppress the growth of P. carinii in vitro. The efficacy o f daphnetin in suppressing the the growth of P. carinii in vitro is related to its ability to chelate iron. Background Although there are several drugs and drug combinations for the treatment of Pneumocystis carinii (P. carinii) pne umonia, all drugs have the toxicity as well as low efficacy. Iron chelators have been proposed as a source of new drugs for combating these infections. We hypothesized that iron chelators would suppress the growth of P. carinii by deprivation of the nutritional iron required for growth. In this study, a short-term axenic culture system of P. carinii was established. Daphnetin (7,8-dihydroxycoumarin), a known iron chelator, was demonstrated to exhibit in vitro activity against P. carinii in this system. Methods P. carinii organisms were obtained from the lungs of immunosuppressed rats. The culture system consisted of Iscove Dulbecco Eagle’s Minimum Essential Medium (IMDM), supplemented with S-adenosyl-L-methionine, N-acetylglucosamine, putrescine, L-cysteine, L- glutamine, 2-mercaptoethanol, and fetal bovine serum, and maintained at 37 ° C in 5% 5% O2, at the optimal pH of 8.0. The culture system was used to assess the effect of daphnetin on the proliferation of P. carinii organisms. The ultrastructures of the treated organisms were observed by transmission electron microscopy. Results The number of cysts and trophozoites increased 8- to 9-fold and 11-to 12-fold, respectively, after 10 days of culture. Daphnetin was found to suppress the growth of P. carinii in a dose-dependent manner for concentrations between 1 μmol / L and 20 μmol / L. The inhibitory activity was suppressed by the chelation of daphnetin with ferrous sulfate in a 2: 1 molar ratio, but it was not suppressed by mixing the culture medium with magnesium sulfate. Reduction of P. carinii numbers after treatment with daphnetin correlated with morphological changes in the organisms, as determined by transmission electron microscopy. Conclusions Daphnetin can suppress the growth of P. carinii in vitro. The efficacy of daphnetin in suppressing the growth of P. c arinii in vitro is related to its ability to chelate iron.