Observations on endonuclear bacteria in euglenoid flagellates

Summary

Rod-shaped inclusions in the nuclei and/or cytoplasm of strains ofEuglena spirogyra, Lepocinclis ovum, Strombomonas conspersa andTrachelomonas oblonga var.punctata have been identified as living populations of bacteria by reference to size, shape and ultrastructure. Most strains contain 20–200 bacteria per nucleus. Each bacterial cell is surrounded by a mucilaginous sheath and consists of a double-membraned envelope, dense cytoplasm with small ribosomes, and a filamentous DNA-containing centre. The bacteria multiply to keep pace with euglenoid division but there is no evidence that the euglenoid cells benefit from or are harmed by the association. Attempts to transfer the endonuclear bacteria to “uninfected” strains and species have been unsuccessful, as have attempts to isolate the bacterium into pure culture. The “infected” strain ofEuglena spirogyra cannot fix nitrogen. The possible nature of the euglenoid/bacterium association is discussed.