Authors

Discussion

Motility of Leishmania spp. parasites is essential for survival during host transitions and is important for lifecycle progression. The presence of an anterior flagellum appendage allows parasite movement. This oscillating flagellum creates the force which pulls the promastigote through changing environments. As the environmental conditions of the parasite change, it morphologically transforms to optimise its survival.

During the host transition, from female phlebotomine sandfly vector to mammalian host, the egestion from the sandfly bite is highly enriched in metacyclic promastigotes (86-98%). This is due to the highly viscous promastigote secretory gel (PSG) creating a ‘sieve’ to block earlier lifecycle stages. The details of how the metacyclics are capable of swimming through this substance compared to other stages remains obscure but is fundamentally linked to infectivity and is the focus of our investigation.

We have adapted 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. The data produced can be used to numerically refocus and create a high resolution reconstruction of parasite movement at different length scales. We have tracked both procyclic and metacyclic promastigote parasites over multiple frames to gain information on the swimming patterns of these cells. This revealed how the parasite and host cells interact with each other and their environments in three-dimensions.

Three-dimensional tracking of promastigotes demonstrates stage-specific differences in swimming behaviour in biologically-relevant environments. Using this technique we have revealed that the mammalian-infective promastigotes are more capable of swimming in highly viscous solutions such as PSG.

Additionally, the DIHM technique has allowed us to investigate how the different stages of Leishmania promastigotes are capable of taxis, including whether human-infectious stage parasites can recognise and are drawn toward phagocytic host cells.

DIHM has allowed us to mathematically quantify the relationship between the active bending of the flagellum and cell movements in response to  the PSG environment of the phlebotomine sandfly midgut versus the presence of mammalian host macrophages. These swimming parameters have helped to determine which factors of promastigote movement are essential for it to respond and thrive within distinct environmental pressures.