Although fusobacteria use amino acids and peptides as energy source, it is not known whether they are able to actively transport peptides into the cell. In the present study the tripeptide glutathione was used as a model substance to investigate peptide uptake in Fusobacterium nucleatum subsp. nucleatum. Cells harvested after 2 days of growth on blood agar or in their exponential growth phase in broth were suspended in buffer with glutathione, L-cysteinylglycine and L-cysteine. As a measure of cell uptake, the formation of hydrogen sulfide was followed. Cells from blood agar had a low capacity to form hydrogen sulfide from the tripeptide glutathione and the dipeptide L-cysteinylglycine. However, hydrogen sulfide was formed from L-cysteinylglycine, but not from glutathione or from L-cysteine, by cells grown in broth in such a way that it strongly indicated an active transport of L-cysteinylglycine with a Km of 18 microM. Hydrogen sulfide was efficiently formed from glutathione by cells grown in broth in the presence 1 mM glutathione. In these cells a glycylglycine-dependent L-gamma-glutamyl peptidase activity was induced. It is probable that the efficient utilization of glutathione for hydrogen sulfide formation mirrored the uptake of L-cysteinylglycine after an L-gamma-glutamyl peptidase had split L-glutamate off from glutathione.