Morphological changes in the bacterium Bacteroides melaninogenicus subspecies melaninogenicus isolated from the human mouth and grown in culture without added blood components.

Striking polymorphism in the cellular morphology could be induced by removal of blood components from the liquid growth medium, but the cells of Bacteroides gingivalis and B. melaninogenicus subspecies intermedius did not exhibit polymorphism when grown under these conditions. The major changes observed with light microscopy were an increase in cell size and extreme polymorphism. Electron microscopy of the polymorphic forms of B. melaninogenicus subspecies melaninogenicus strains showed that such cells lacked both the outer cell membrane and peptidoglycan layer. Serum promoted the growth of these strains, suggesting that some blood component is either conducive to the synthesis of the cell wall or masks an unknown inhibitor for cell-wall synthesis contained in the medium.