the three domains of life are bacteria, archaea, and eukarya. bacteria and archaea are prokaryotic and have only one cell. bacteria range in size from 0.15 cubic micrometres ("mycoplasma") to 200,000,000 cubic micrometres ("thiomargarita namibiensis"). bacteria have shapes which are useful in classification, such as round, long and thin, and spiral. some bacteria cause diseases. bacteria in our intestines are part of our gut flora. they break down some of our food. both bacteria and archaea may live where larger forms of life cannot. bacteria have a molecule called peptidoglycan in their cell wall, but archaea do not. archaea have a molecule called isoprene in their cell membrane, but bacteria do not.