Two aeromonads: Growth of symbionts fromHydra viridis

A large population of bacteria resides in the gastrodermal and ovarian tissue of the freshwater green coelenterateHydra viridis (Ohio, Jubilee, and Carolina strains). The intracellular bacteria are strongly correlated with the presence of symbiotic chlorellae in the animal cells. The bacteria accompany the chlorellae when they are expelled in vesicles during stages of sexual maturation. Two isolates of these bacteria were taken from ruptured bacterial-algal vesicles flushed out of the enteron of surface-sterilzed hydras. They were cultured on proteose peptone. Both were identified asAeromonas punctata, on the basis of over forty traits. These large, Gram-negative rods differ very little from published characteristics ofA. punctata subsp.punctata. UnlikeA. punctata subsp.punctata, the hydra symbionts grew on KCN broth and lacked the lysine decarboxylase reaction. We identified the two isolates as members of the same new strain ofAeromonas punctata symbiotic inHydra viridis. A different aeromonad could not be isolated from vesicles flushed out of surface-sterilized hydras, but it appeared along with the first aeromonad on plates from which smears of the holdfast and hypostome region of ethanol-rinsed hydra were made. This second orange-pigmented bacterium in a member of the holdfast microbial community associated with hydra and hydra eggs. It differed fromAeromonas hydrophila subsp.anaerogenes in only 5 out of over 40 traits tested. All differences were losses of metabolic activities. Both hydra-associatedA. hydrophila andA. punctata, which are easily grown, and yet form regular and natural associations with the greenHydra viridis, may prove useful for understanding metabolic relationships in micro-organisms adapted for symbiotic associations.