unicellular prokaryotic microorganisms which generally possess rigid cell walls, multiply by cell division, and exhibit three principal forms, round or coccal, rodlike or bacillary, and spiral or spirochetal; bacteria can be classifed by their response to oxygen: aerobic, anerobic, or facultatively anerobic; by the mode by which they obtain energy: chemotrophic or phototrophic; for chemotrophs by their source of chemical energy: lithotrophic (from inorganic compounds) or organotrophic (from organic compounds); and by where they get their carbon: heterotrophic (from organic sources) or autotrophic (from carbon dioxide); they can also be classifed by whether or not they stain, based on the structure of their cell walls with crystal violet dye: gram-negative or gram-positive.