Cells of Escherichia coli swim either end forward.

Chemotactic cells of the bacterium Escherichia coli were marked asymmetrically by growth on a rich medium containing tetrazolium red. When this dye is reduced, it tends to form a refractile granule near one end of the cell, readily visualized by dark-field microscopy. In smooth-swimming cells, the marker was found with equal probability in front or behind. In wild-type cells, tumbles changed the cell orientation nearly as often as not. Some cells formed flagellar bundles at one end more frequently than at the other, but the run-interval distributions were the same either way. We conclude that the sensory system does not favor one end of the cell over the other. Thus, chemoreceptors that appear in patches at only one pole do not serve as a nose.