The bacterial flagellum is a large biomolecular assembly used by many types of bacteria as a helical propeller for forward swimming and turning. The flagellum is remarkable in that its properties differ greatly depending on the direction in which it is rotated, allowing the bacterium to switch between swimming straight ("running") and turning ("tumbling"). The mechanics of the flagellum are of interest both to biologists and mechanical engineers. The molecular mechanisms of the transition in the flagellum between running and tumbling modes is unknown. Because of the flagellum's size (several micrometers in length) and composition (made up of 30,000 protein subunits) it presents a challenge to computational modeling. Researchers have now achieved an advance describing the flagellum in both its running and tumbling state. For this purpose, the researchers developed a computational model of the system that glosses over atomic level detail, but resolves the shapes of all proteins making up a bacterial flagellum, simulating a simplified version of the system using the program NAMD. The results, reported recently, showed that the flagellum's transition between swimming straight and tumbling is triggered by friction due to the water around the bacterium. More information on the flagellum project can be found here.