The microscope images of coronavirus reveal they have ellipsoidal shapes. The scientists modeled these different shapes to see how it impacts the speed that the particles rotate. Credit: OIST
In the early days of 2020, we didn’t know much about SARS-CoV-2—except what it looked like. The gray sphere with red spikes quickly became a staple across our televisions, phones, computer screens and newspapers.
But now, a global team comprising researchers from Queen’s University (Canada) and Okinawa Institute of Science and Technology (OIST, Japan) say we’re picturing SARS-CoV-2 incorrectly. Not only that but its real shape contributes to how easily the virus can be transmitted. The researchers say microscope images of infected tissues reveal that SARS-CoV-2 particles are actually ellipsoidal, displaying a wide variety of squashed and elongated shapes.
Fluid dynamics
As early as April 2020, scientists worked out that two specific cell types in the nose are the likely initial infection points for SARS-CoV-2. According to their paper, the location of the goblet cells and ciliated cells on the surface of the inside of the nose make them highly accessible to the virus—and also assist transmission to others.
That’s why the OIST team took on this research from a fluids perspective, rather than chemical or biological.
“When coronavirus particles are inhaled, these particles move around within the passageways in the nose and lungs,” said study author Eliot Fried, who leads the Mechanics and Materials Unit at OIST. “We [studied] to what extent they are mobile in these environments.”
The specific type of movement the scientists modeled is called rotational diffusivity, which determines the rate at which particles rotate as they move through fluid—in this case, droplets of saliva. Particles that are smoother and more hydrodynamic encounter less drag resistance from the fluid and rotate faster. For SARS-CoV-2 particles, this rotational speed affects how well the virus can attach to and infect cells.
“If the particles rotate too much, they might not spend enough time interacting with the cell to infect it, and if they rotate too little, they might not be able to interact in the necessary way,” explained Fried.
In their study, published in Physics of Fluids, the scientists modeled both prolate and oblate ellipsoids of revolution. These shapes differ from spheres in the length of their axes. Spheres have three axes of identical length, prolate shapes have one longer axis, and oblate shapes have one shorter axis.
The study results show that the more a particle differs from a spherical shape, the slower it rotates. The scientists say this could mean that the particles are better able to align and attach to cells—making transmission easier.
Previous research from the same team showed that the presence of triangular-shaped spike proteins lowers the speed at which the coronavirus particles rotate, also increasing their ability to infect cells. So, for this model, the team added each spike protein, represented by a single sphere, onto the surface of the ellipsoids to create the most realistic model yet.
“We figured out the arrangement of the spikes on the surface of each ellipsoidal shape by assuming that they all contain the same charge,” said study author Vikash Chaurasia, a postdoctoral researcher at OIST. “Spikes with identical charges repel each other and prefer to be as far from each other as possible. They therefore end up evenly distributed across the particle in a way that minimizes this repulsion.”