Cultivation of bacteria-free Hydra viridis: missing budding factor in nonsymbiotic hydra.

Bacteria-free hydra, cultured in sterile media, were fed bacteria-free larvae of Artemia salina. Normal growth and budding were obtained in symbiotic and aposymbiotic Hydra viridis. Two nonsymbiotic hydra species did not form buds under bacteria-free conditions. When these hydra were fed nonsterile Artemia, or if the medium was reinoculated with bacteria isolated from budding stock cultures, normal budding was resumed. An exogenous budding factor, which can be provided by nonsterile Artemia larvae, or even by some bacteria, appears to be required by these nonsymbiotic hydra. This factor is endogenous in Hydra viridis.