Trypanosoma Swims Like an Active Corkscrew

Trypanosoma Brucei is a unicellular eukaryotic parasite that causes sleeping sickness in cattle and humans, impairing the economy and public health of sub-Sahara Africa. During its life cycle, Trypanosoma navigates through complex and distinct environments, ranging from the narrow gut of tsetse flies to the crowded blood vessels of mammals. Despite some modifications at different stages, their general morphology and swimming motion are conserved: a tapered slender body performs corkscrew-like motion led by the thin, anterior end, driven by a single flagellum attached to the cell membrane. Such swimming traits are shared by many other unicellular parasites that cause tropical diseases, or even insect-infecting parasites, such as Crithidia Fasciculata. However, due to its rapid motion and complex body deformation, conventional optical microscopy is not fast enough to fully reveal the dynamics in 3D, leaving the exact swimming mechanism in debate.

In this talk, I will show our recent experimental and simulation results which unveil surprising features of their swimming behavior. We attach fluorescent particles to different body parts of swimming Trypanosoma and obtain their instantaneous 3D locations by comparing their defocused images with pre-established libraries. The particle trajectories indicate that TB exhibits a unique local motion consisting of three distinct components: unidirectional rotation, azimuthal oscillation, and bending. The amplitudes of both the azimuthal oscillation and bending are large at the leading tip and decrease at the body part. The phase difference between azimuthal oscillation and bending remains at pi/2 throughout, resulting in a clockwise rotation of the flagella (viewing from behind) and propagation of a right-handed helical wave from the thin anterior to the thick posterior end. The body, although having a right-handed helical shape, undergoes a counterclockwise rotation. These results contradict previous findings, but were confirmed by our numerical simulation using regularized Stokeslet method. The unusual dynamics are explained by conservation of angular momentum and local activation modes of the attached flagellum complex. The exact biomechanical mechanism of flagellum activation remains unknown at this stage.