A lot of my research looks at how to move particles around, particles about 100 nanometres across. For example when we made stratified coatings we did this with a mixture of particles of sizes 60 and 400 nanometres. The corona virus (see above) is about 100 nanometres across. Viruses are barely alive. They have no metabolism themselves, no more than the inert colloidal particles my colleagues and I study. The virus above has the genetic material in the centre, surrounded by a protein coat. The red things are its spike proteins, that help it enter and infect a cell. The colours above are false, I think it is reconstructed from electron (not light) microscopy.
Now I only know anything about moving things in liquids, and over distances of up a millimetre at most — we scientists specialise. But viruses may need to move over this sort of distance.
If a virus arrives at the lining of your airway, it arrives in the liquid layer lining the airway. Very roughly speaking along the inside of your airways I think there are basically two liquid layers, one on top of another. The top layer is of mucus, and the bottom layer is a less sticky layer containing tiny cilia which wiggle back and fore continuously moving the top layer of mucus up and out. The sticky mucus can then trap nasties like bacteria and viruses, and which are then moved up and away from the lungs. The two layers are each around 10 micrometres, or 0.01 millimetres thick.
I couldn’t find studies of how the corona virus moves through mucus, but people have taken a look at how the Ebola virus moves in mucus*. In mucus, a significant fraction of the Ebola virus particles moved a few micrometres in about a second. This is about as fast as they move in pure water, despite mucus being a sticky gel-like substance, If these viruses can maintain this mobility for about 10 minutes**, they could cross the layer of mucus.
This fast motion surprised me. My naive picture of a viscous (i.e., sticky, thick) liquid like mucus, is that the stickiness comes from a network — the network makes the liquid reluctant to flow by resisting forces on it. Any network strong enough to make a liquid viscous should have a typical distance between the network strands of less than 100 nanometres, and so should tend to trap both corona and Ebola. Both viruses are at least as big as the distance between strands. But apparently this trapping doesn’t happen***.
The rapid motion of viruses in mucus seems odd at first sight, but maybe I am misinterpreting the role of mucus. Button et al showed that the layer below the mucus is impermeable to particles the same size as a virus, so maybe mucus soaks up the viruses and then virus-containing mucus is expelled, while it is the layer beneath that is the barrier.
Viruses have been evolving to better infect animals, and in turn animals have been evolving (for example their mucus) to fight viruses for many many millions of years. It is difficult for us to come along, long at the end point of millions of years of evolution, look at the very complex result of this evolution, and work out what is going on. But I guess one outcome of the current terrible situation we are in, is that it will motivate spending money into researching exactly how viruses get into our bodies and infect us.
* The ebola virus is a very different shape to that of the corona virus. Instead of being roughly spherical and 100 nanometres across as the corona virus is, it is long and roughly cylindrical with a diameter of about 10 nanometres and up to 1000 nanometres long. This
** Note that the time to diffuse a distance, increases as the square of the distance. Also, in this case some virions much more slowly, i.e., are stuck, and it is possible that the more mobile virions get stuck after a time. The experiments tracked virions only over a few seconds, so can’t answer this question.
*** Particles 100 nanometres across whose surface had a inert polymer (called PEG) on their surface mostly diffused pretty freely, but ones whose surface was highly charged appeared to stick to the mucins, and these were immobilised. Mucus is, like pretty much everything in biology, complex. For example, on microscopic length scales it appears to be very non-uniform. On the length scales we can see, maybe down to a millimetre, mucus looks uniform but this is not what a 100 nanometre virions see. They presumably see an environment that varies a lot from one point to another. Maybe the mesh is very weak in parts.