Swim Like Your Lifecycle Depends On It. Investigating Motility of Leishmania mexicana; its Impact on Parasite Lifecycle Progression and Infectivity.

The motility of Leishmania promastigote parasites is important for survival during host transitions and lifecycle progression. An oscillating flagellum at the anterior end of the promastigotes pulls it through environmental conditions which change significantly during the lifecycle. The parasite morphologically transforms to optimise infection potential. This study adapts a unique method of high-speed, three-dimensional imaging called digital inline holographic microscopy (DIHM) allowing us to examine the movements of Leishmania mexicana promastigotes. We have tracked distinct stages of promastigote parasites over multiple frames to gain information on the swimming patterns of these cells. Quantification of the 3D trajectories reveals stage-specific differences in swimming behaviour. Using this technique we reveal that mammalian-infective metacyclic promastigotes are more capable of swimming in highly viscous solutions, a result which has interesting implications in the ability of this specific stage to transmit through promastigote secretory gel. Additionally, the DIHM technique has allowed us to investigate whether these different stages of Leishmania promastigote are capable of sensing chemicals in their environment. We reveal how distinct chemotaxic capabilities could play a role in the uptake of parasites by host cells during early infection. Mathematically quantifying the cell movements of L. mexicana within contrasting, biologically relevant environments has revealed swimming mechanisms that are essential for the parasite to remain unencumbered by environmental pressures and adapt their motility to reach preferred conditions.