PARASITE MOTILITY

Motion is essential for immune escape and tsetse passage - and it is a very complex business.

We found that the trypanosome flagellum can productively beat from both ends. This is unique in nature and essential for parasite virulence.

The trypanosome's journey through the tsetse fly takes several weeks.

Studies on the motility of microorganisms have focused on relatively few model organisms, including the bacterium E. coli, eukaryotic spermatozoa or the small green algae Chlamydomonas. The seminal work on bacterial motion in particular has been fundamental to teaching us about the swimming of single cells at low Reynolds numbers. More recently the formation of spatiotemporal patterns, e.g. the self-organisation of microswimmers into dynamic vortices and in the vicinity of planar surfaces, has been reported. Large-scale coordination of cells may well be mediated by hydrodynamic interactions that do not depend on the exchange of chemical signals.

As an adaptation to the varying microenvironments within the mammalian host, trypanosomes modulate the beat direction of their flagellum in response to purely biomechanical cues. The host microenvironment thus determines in which direction the trypanosome flagellum beats, and hence in which direction the cell moves. In the human bloodstream, the deadly parasites are precisely biomechanically adapted to the presence of blood cells and exhibit very rapid forward movement, which is essential for clearing host antibodies from the cell surface. Under more rigorous confinement the flagellar beat reverses and the cells swim backwards, thereby avoiding getting trapped (e.g. in tissue spaces). In the absence of any confinement, beat reversal occurs frequently and randomly, which retards motion. The concept of pathogens biomechanically exploiting the host microenvironment for efficient motility and hence for persistence is interesting, as is the triggering of flagellar beat reversal under confinement.

Trypanosomes also swim in the tsetse fly. We found that depending on their location and developmental stage, the unicellular parasites move either singly or in huge swarms. They display collective behaviour and can congregate or navigate in strong confinement. The tsetse-trypanosome system is thus ideally suited for a systems biophysical exploration of microbial motion.