Abstract: 
Bacterial chemotaxis has been widely studied from both the microscopic and macroscopic points of view. In this talk, I will present a recent work (joint with Zahra Aminzare) on the collective behavior of E. coli in response to two stimuli. We derive from a microscopic model of the behavior of individuals a macroscopic model that describes the population-level behavior of bacteria. In particular, the connection between two different levels of description enables us to understand better a realistic model of E. coli chemotaxis. By using our macroscopic model, we show analytically that the response of E. coli to two stimuli depends on the ratio of the respective receptor numbers. We also examine our theory with  Monte Carlo agent-based simulations. Moreover, these results are consistent to experimental results (Y. Kalinin, et al., 2010) on responses of E. coli to two stimuli such as Serine and Asparate.