E. Coli and Salmonella generate propulsion by rotating flexible helical flagella.  There are typically about six to ten flagella, rotated by motors embedded at random points on the cell body.  The chemotaxis strategy of these bacteria is to trace out a directed random walk with a drift velocity in the direction of increasing chemoattractant.  During a step of the random walk, a bacterium ``runs," or swims in a directed manner.  At the end of each step, the bacterium tumbles to randomize its direction.  During a run, the flagella come together and form a bundle.  Our macroscopic experiments show how the viscous flows set up by two rotating helices lead to bundling.