The members of the domain Bacteria are unicellular and prokaryotic. Their cells have thick, rigid walls that surround a cell membrane. The cell walls contain a substance know as peptidoglycan. The domain Bacteria corresponds to the kingdom Eubacteria. These bacteria are ecologically diverse, ranging from free-living soil organisms to deadly parasites. Some photosynthesize, while other do not. Some need oxygen to survive, while others are killed by oxygen.