Motility enables the bacterium to move toward a favorable environment or away from a particular stimulus called taxis . Chemotaxis (include chemicals) and phototaxis (include light). Bacteria do not always swim aimlessly but are attracted by such nutrients as sugars and amino acids, and are repelled by many harmful substances and bacterial waste products. Movement toward chemical attractants and away from repellents is known as chemotaxis. The mechanism ofchemotaxis in E.coli has been studied most. Forward swimming is due to counterclockwise rotation of the flagellum, whereas tumbling results from clockwise rotation. The bacteria must be able to avoid toxic substances and collect in nutrient-rich regions and at the proper oxygen levels. E.coli has four different chemoreceptors that recognize serine, aspartate and maltose, ribose and galactose and dipeptides respectively. These chemoreceptors often are called methyl-accepting chemotaxis proteins (MCPs)

Some bacteria can move bymechanisms other than flagellar rotation. Spirochetes are a group of bacteria that have unique structure and motility (Treponemapallidum, the causative agent of syphilis and Borrelia burgdorgeri , the causative agent of Lyme disease).Spirochetes travel through viscous substances such as mucus or mud by flexing and spinning movement caused by special a xial filaments - bundles of fibrils that arise at the ends of the cell beneath the outer sheath and spiral around the cell (fig. 18).The rotation of the filaments produces an opposing movement of the outer sheath that propels the spirochetes by causing them to move like corkscrews.