PHYSICAL MATHEMATICS SEMINAR TOPIC: CHIRALITY TRANSFORMATIONS PROPAGATING ON BACTERIAL FLAGELLA SPEAKER: GREG HUBER Physics Department University of Massachusetts, Boston ABSTRACT: Chemotaxis in many bacterial species is made possible by the remarkable dynamics of their multiple, rotating, helical flagella. They bundle and de-bundle as their rotary motors episodically change rotational direction. When the flagella are bundled, the bacterium moves linearly, but the Dissolution of the bundle leads to a tumbling event that effectively randomizes the cell's orientation. The motor reversal that initiates the tumbling not only torques the flagella oppositely, but also reverse the chirality of the filament, turning a left-handed helix into a right-handed helix. Hotani has performed careful experiments on helical flagella in external flows and he observed that regions within the filament periodically flip to the opposite chirality, andthat those domains propagate stably downstream. I'll present a dynamical model for this phenomenon based on the existence of two competing locally stable states of opposite chirality whose interaction with the flow is through the torque they produce. The model displays a number of the key features seen in the experiments. DATE: TUESDAY, NOVEMBER 27, 2001 TIME: 2:30 PM LOCATION: Building 2, Room 338 Refreshments will be served at 3:30 PM in Room 2-349 Massachusetts Institute of Technology Department of Mathematics Cambridge, MA 02139