Bacteria are the smallest, single-cell microorganisms (prokaryotes). They reproduce by binary fission. Although bacteria have a nucleus-equivalent genome (nucleoid) and an independent machinery for protein biosynthesis (ribosomes), they do not have a real nucleus (no double membrane) and usually lack organelles. They can be devided in several classes. Related anaerobic bacteria are: clostridium difficile, clostridium perfringens, the causative organism of gas gangrene, clostridium tetani, the causative organism of tetanus, and clostridium botulinum, the causative organism of botulism.