 E. coli is a gram @-@ negative , facultative anaerobic ( that makes ATP by aerobic respiration if oxygen is present , but is capable of switching to fermentation or anaerobic respiration if oxygen is absent ) and <unk> bacterium . Cells are typically rod @-@ shaped , and are about 2 @.@ 0 micrometers ( μm ) long and 0 @.@ 25 – 1 @.@ 0 μm in diameter , with a cell volume of 0 @.@ 6 – 0 @.@ 7 <unk> . 
