Malaria parasites form whirlpools – ScienceDaily

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Malaria disease is triggered by single-celled parasites that accumulate in large groups in the salivary glands of mosquitoes before being transmitted to humans. The limited space there prevents them from actually moving, unless this restriction is lifted by means of suitable experimental preparation. In such experiments, researchers from the University of Heidelberg set the pathogens in motion and analyzed the acquired image data using state-of-the-art image processing methods. The data show that collectively moving pathogens form vortex systems largely determined by physical principles. Special computer simulations made it possible to identify the mechanisms underlying these rotational movements.

The collective movement of biological organisms is a common occurrence in the natural world. Insects and fish, for example, tend to move in swarms. Often collective movement also occurs at the cellular level, such as when cancer cells migrate from a tumor or bacteria form a biofilm. The collaboration of many individuals can give rise to what is called emergent behavior – new characteristics that would not otherwise exist in this form. “In physics, the community creates such important processes as phase transitions, superconductivity and magnetic properties,” explains Professor Ulrich Schwarz, head of the “Physics of Complex Biosystems” working group at the Institute for Theoretical Physics in the University of Heidelberg. In an interdisciplinary study in collaboration with Prof. Dr Friedrich Frischknecht (malaria research) and Prof. Dr. Karl Rohr (biomedical image analysis), he showed that collective movement can also occur in Plasmodium, the causative agent of malaria.

The single-celled organism is injected into the skin by a mosquito bite, growing first in the liver and then later in the blood. Because Plasmodium acts as a single cell in most of its stages, so far its collective properties have hardly been studied. In the mosquito’s salivary gland, the parasite has a long, curved shape, similar to a crescent moon, and is known as a sporozoite. “As soon as the sporozoites are injected into the skin by the mosquito, the individual parasites begin to travel rapidly to the blood vessels. This is the critical phase of the infection, as it is only successful if a pathogen reaches the bloodstream,” emphasizes Prof. Frischknecht.

In their studies at the Center for Infectious Diseases at the University Hospital Heidelberg, Friedrich Frischknecht and his team found that parasites from infected salivary glands can be collectively mobilized. To do this, the salivary glands are dissected from the mosquito and carefully pressed between two small glass plates. The researchers were surprised to find that the crescent-shaped cells form rotating vortices in the new preparation. They are reminiscent of the collective movements of bacteria or fish, although they differ in that they always turn in the same direction. The parasitic vortices therefore have a chiral character and, also unexpectedly, a fluctuating size. According to Professor Frischknecht, these oscillations indicate emergent characteristics, as they are only possible in the collective of moving cells and are reinforced in larger vortices.

To understand these phenomena more precisely, the experimental data were analyzed quantitatively. The groups of Ulrich Schwarz and Karl Rohr, head of the Biomedical Computer Vision Group at the BioQuant Center of the University of Heidelberg, used state-of-the-art image processing methods for this purpose. They were able to track the individual parasites in the spinning vortices and measure both their speed and curvature. Using so-called agent-based computer simulations, it was possible to identify precisely the laws that can explain all aspects of the experimental observations. The interplay of active motion, curved cell shape, and chirality in conjunction with mechanical flexibility is sufficient to explain the sorting and oscillation phenomena in parasitic vortices. The oscillations observed by scientists arise because the movement of individual pathogens is converted into elastic energy which is stored in the vortex. “Our new model system offers the possibility to better understand the physics of collectives with elastic properties and perhaps to make them usable for technical applications in the future,” says physicist Ulrich Schwarz.

In the next step, the researchers will investigate exactly how the chirality of motion occurs. The structure of sporozoites suggests different possibilities that can be investigated in experiments with genetic mutations. Early computer simulations have already shown that right- and left-turning parasites rapidly separate and generate separate vortex systems. A better understanding of the underlying molecular mechanisms could open new avenues to disrupt sporozoite movement at the onset of every malaria infection. “In any case, our study has shown that the mechanics of pathogens plays an extremely important and hitherto neglected role, a finding that also opens up new perspectives for medical interventions,” explains infectiologist Friedrich Frischknecht.

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Material provided by University of Heidelberg. Note: Content may be edited for style and length.

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