Effect of Viscosity on Leishmania mexicana motility

Motility is important for the Leishmania parasite to survive, transition into the next host and differentiate into the mammalian infective form. There are several external factors that this parasite faces in its environment that could affect its movement. This research examined the impact of changes in viscosity on motility of L. mexicana, the causative organism for cutaneous leishmaniasis. Velocity, curvature, and helicity were used to describe observed motility. Known molecular markers were used to validate procyclic and metacyclic promastigote life cycle stages. Individual cell movement was tracked in 3D with digital inline holographic microscopy. Finally, polymer and fPPG samples that I carefully prepared were used to measure their viscosity using magnetic tweezers by L.G. Wilson.
In support of previous findings by Findlay et al (eLife, 2021), this study found that the human infective metacyclic promastigote cells swim faster in a ‘run and tumble’ behaviour, whereas the procyclic promastigote cells swim slower in a helical meandering pattern. Metacyclic promastigote cells display a lower swimming curvature compared to procyclic promastigote cells at comparable viscosities. Helicity was found to have an equitable, symmetrical distribution in both procyclic and metacyclic promastigote cells, without evident bias towards right or left handedness. Overall, these trends were repeated across different polymer and fPPG concentrations. The absolute viscosity measurements in Ficoll, PVP, and fPPG were comparable, showing an increase with rise in concentration.
The investigation of L. mexicana cell movement in fPPG, a biologically relevant environment, identified swimming mechanisms critical for the parasite to navigate and adapt to environmental constraints. Such pre-adaptation allows these parasites to alter their motility and maximise infection potential, promoting life cycle progression.