Unicellular Parasite Motility: A Quantitative Perspective

Abstract

The question of how single cells swim is of primary medical importance - especially in the case of pathogenic parasites. Biochemical and cell biological studies have helped elucidate many of the protein building blocks, and chemical interactions involved in motility. However, a complete understanding of microswimmers necessitates a physical and quantitative understanding of swimming mechanisms. In this context, the motility of the parasite Trypanosoma brucei brucei is characterized in biomimetic environments. Trypanosomes, unicellular parasites, cause deadly diseases in humans and cattle in Africa and South America. They are transferred to a mammalian host through an insect vector and thrive within the blood stream and eventually invade the central nervous system. The parasite propels itself through these diverse environments with the aid of a flagellum. In a minimal homogeneous nutrient rich environment, cells exhibit one of three motility modes distinguishable by their directional persistence. Directional cells take on a straighter shape, while cells that exhibit little net displacement appear more bent. Ascribing the cell body to a worm like chain, we use the cell end to end distance (from base to tip) as a measure of cell stiffness and find that the elongated shape associated with higher directionality is also correlated with higher stiffness. Cell trajectories show a persistence in average swimming direction on the order of 15 s. Further, correlation analysis using high speed microscopy data of 1 kHz uncovered an additional relaxation time arising from strong body distortions in the range of 20 to 100 ms. Random walk models are formulated to describe the motility modes as well as the fast distortions of the cell body. In polymer networks and more viscous environments such as those found in the extracellular matrix, trypanosomes swimming speed is reduced. However, some directionally persistent trypanosomes are found to tunnel their way through networks with mesh sizes smaller than the diameter of the cell. Other cells show little net movement as shown by scaling analysis and appear to probe the elasticity of the network. We show that the movement of these cells can be used to describe the relative differences in elasticity of actin and collagen networks towards a new concept of active microrheology . Trypanosomes are found to exhibit a propensity to swim close to containing boundary walls and are highly aligned to these boundaries. Using microfluidic channels, trypanosomes suspended in culture medium subjected to flow experience a lift force away from vessel walls and migrate to the center. Purely hydrodynamic effects arising from the trypanosome s shape and density are distinguished from effects of cell motility by comparing with immobilized trypanosome behaviour. We find that the most striking differences in the behaviour between live and immobilized cells arise at flow velocities below 0.1 mm/s (more than ten times the self propelling speed of trypanosomes). In this range of resulting shear stresses, trypanosomes exhibit a velocity dependent oscillatory motion swimming upstream from one side of the channel to the other. Immobilized cells tumble in flow, and unlike active cells, do not exhibit an orientational preference. Replacing the suspending medium with whole blood, does not result in significant differences in the center of mass distribution of trypanosomes. However utilizing a constriction-expansion geometry to mimic the cell free layer near the blood vessel walls, we demonstrate that like white blood cells, trypanosomes are expelled by red blood cells toward the boundaries due to differences in cell stiffness. These studies are pertinent to our understanding of how trypanosomes are able to approach vessel walls and invade membrane barriers, including the blood brain barrier for entry into the central nervous system, despite the high shear stresses of blood flow. The present work demonstrates that a quantitative, physical approach uncovers fascinating details of low Reynolds number swimmers.