Rotational-translational models for interpretation of quasi-elastic light scattering spectra of motile bacteria.

Quasi-elastic light scattering is a powerful tool for studying dynamic behavior of motile bacteria in liquid suspension. Previously we proposed an ellipsoidal structural model to account for the straight line motion of E. coli bacteria. We have in this article extended our structural model to include rotational motions superimposed on the translational motion of the center of mass. A general formulation is given by which the light scattering spectrum of any rotational-translational motion of a structured particle can be calculated. Specific calculations are carried out for three types of rotational motions, namely, planar wiggling motion, pure rotation, and helical rotation, around the average direction of motion. Further narrowing of the scattered field correlation function as compared with that of the straight line motion is observed in all three cases. We finally compare the helical rotation model with our previous experimental results for E. coli and find a substantial improvement over that of the straight line motion model.