bacteria - a large group of unicellular or multicellular organisms, lacking chlorophyll, multiplying rapidly by simple fission, some species developing a highly resistant resting ("spore") phase. They are the principle members of the prokaryotes, whose genetic material is not packaged into chromosomes contained in nucleus. Instead they have a single circle of DNA free in the cytoplasm, which reproduces by a "rolling circle" mechanism. They lack mitochondria, oxidative phosphorylation, occuring across the plasma membrane, and they are bounded by a cell wall situated outside and separated from the plasma membrane by a periplasmic space ... Since Pasteur's time they have been actively studied and classified in relation to disease but they are also important in the production of chemicals, enzymes and antibiotics., üherakuline tuumata (prokarüootiline) mikroorganism