The ellipsoid tracer inside the E-coli suspension shows a diffusive motion at long time scales (~1 sec), as the tracer is being pushed around by flows generated by bacteria swimming. At low bacteria concentration, the tracer center of mass tends to move along the major axis (blue trajectory in figure on the left). At higher concentration where the bacteria display 'flocking' behavior, the tracer center of mass tends to move along the minor axis (red trajectory in figure on the left). By relating to a simple one dimensional diffusion along the angular direction, our model explains the observation by explicit consideration of the straining field generated by the bacteria.