In order to characterize the delayed effect of clindamycin and macrolide antibiotics against Toxoplasma gondii tachyzoites (E. R. Pfefferkorn and S. E. Borotz, Antimicrob. Agents Chemother. 38:31-37, 1994), we have carefully examined the replication of parasites as a function of time after drug addition. Intracellular tachyzoites treated with up to 20 M clindamycin (>1,000 times the 50% inhibitory concentration) exhibit doubling times indistinguishable from those of controls (ϳ7 h). Drug-treated parasites emerge from infected cells and establish parasitophorous vacuoles inside new host cells as efficiently as untreated controls, but replication within the second vacuole is dramatically slowed. Growth inhibition in the second vacuole does not require continued presence of drug, but it is dependent solely on the concentration and duration of drug treatment in the first (previous) vacuole. The susceptibility of intracellular parasites to nanomolar concentrations of clindamycin contrasts with that of extracellular tachyzoites, which are completely resistant to treatment, even through several cycles of subsequent intracellular replication. This peculiar phenotype, in which drug effects are observed only in the second infectious cycle, also characterizes azithromycin and chloramphenicol treatment, but not treatment with cycloheximide, tetracycline, or anisomycin. These findings provide new insights into the mode of clindamycin and macrolide action against T. gondii, although the relevant target for their action remains unknown.The protozoan parasite Toxoplasma gondii is a ubiquitous human pathogen long recognized as a source of congenital neurological abnormalities (19). In recent years, this parasite has also acquired considerable notoriety as an opportunistic infection associated with AIDS (16). The ability of T. gondii parasites to persist as latent cysts in the tissues of infected patients mandates chronic treatment for infected AIDS patients, to guard against recrudescence. Unfortunately, the traditional therapeutic regimen of pyrimethamine plus sulfonamides (18) is not always suitable for prolonged treatment, because of the emergence of sulfa hypersensitivity and other adverse side effects (12,14,27).Clindamycin (a lincosamide) and several macrolide antibiotics have proven effective for the treatment of AIDS-toxoplasmosis, usually in combination with pyrimethamine (6, 13, 15). These compounds are known to block protein synthesis in bacteria by interacting with the peptidyl transferase domain of 23S rRNA (5), but their target in T. gondii and related parasites remains unclear (1). Early difficulties in establishing a functional in vitro system to study clindamycin and macrolide action against T. gondii (4, 9, 17) can now be explained by the long lag period between drug administration and effect (20,21). Nanomolar drug concentrations block parasite replication, but only 2 to 3 days after treatment-a remarkable delay, considering that the tachyzoite undergoes ϳ8 generations in this time.In order to more precisely e...

The purpose of this study was to determine whether Strongyloides stercoralis FKTF-1, a transcription factor of the FOXO/FKH family and the likely output of insulin/IGF signal transduction in that parasite, has the same or similar developmental regulatory capabilities as DAF-16, its structural ortholog in Caenorhabditis elegans. To this end, both splice variants of the fktf-1 message were expressed under the control of the daf-16α promoter in C. elegans carrying loss of function mutations in both daf-2 (the insulin/IGF receptor kinase) and daf-16. Under wellfed culture conditions the majority (91%) of untransformed daf-2; daf-16 double mutants developed via the continuous reproductive cycle, whereas under the same conditions 100% of daf-2 single mutants formed dauers. Transgenic daf-2; daf-16 individuals expressing fktf-1b showed a reversal of the double mutant phenotype with 75% of the population forming dauers under well-fed conditions. This phenotype was even more pronounced than that of daf-2; daf-16 mutants transformed with a homologous rescuing construct, daf-16α::daf-16a (56% dauers under well fed conditions), indicating that S. stercoralis fktf-1b can almost fully rescue loss-of-function mutants in C. elegans daf-16. By contrast, daf-2; daf-16 mutants expressing S. stercoralis fktf-1a, encoding the second splice variant of FKTF-1, showed a predominantly continuous pattern of development identical to that of the parental double mutant stock. This indicates that, unlike FKTF-1b, the S. stercoralis transcription factor FKTF-1a cannot trigger the shift to dauer-specific gene expression in C. elegans.

No abstract

No abstract

No abstract