Swimming bacteria with helical flagella are self-propelled micro-swimmers in nature, and the swimming strategies of such bacteria vary depending on the number and the position of flagella on the cell body. In this talk, we will introduce two microorganisms, multi-flagellated E. coli and single-flagellated Vibrio A. The Kirchhoff rod theory is used to model the elastic helical flagellum and the penalty method is employed to describe the dynamics of the rigid cell body. The hydrodynamic interaction between the fluid and the cell is represented by the regularized Stokes formulation. The focus of the talk will be on how bacteria reorient swimming direction.