How do viruses move outside the cell?

In line with the recent article “Are viruses alive?” I would like to further explore the general nature of viruses. One question that I was recently asked was “how does a virus move?”

Being that viruses are not technically alive in the sense that we know it they also cannot move in a self-directed manner. This is in stark comparison to some other microbes such as Schistosoma cercariae, a parasitic worm, which is capable of burrowing through intact human skin and gaining access to the vascular system within 5 minutes (1).

Thankfully viruses cannot do this, much to our benefit. Because of how they are constructed, viruses cannot mechanically move in a self-directed manner and are subject to movement solely based upon environmental interactions. Essentially, they are not only hijackers who take over cellular processes for their own good, but environmental hitchhikers as well.

One prime example of “hitchhiking” is used by the influenza virus, the causative agent of the common flu that we all know so well. This virus infects the upper airway and lungs and must come into intimate contact with these surfaces to initiate an infection. Since it cannot get directly from one host to another it hitchhikes in the tiny particles of saliva and mucous produced when you sneeze or cough. If you inhale another infected person’s particles it allows the influenza virus direct access to your lung tissue. It can also spread by contacting contaminated surfaces (fomites) and then touching your face, effectively inoculating yourself with the virus.

A second example of this is the transmission of poliovirus. This virus infects the intestinal tract and is shed in very large amounts in feces. If a person comes into contact with even trace amounts of these contaminated feces and ingests them a new productive infection begins.

This characteristic of viruses is highly advantageous for researchers as it facilitated the use of the viral plaque assay; one of the standard workhorses of the microbiological laboratory.

Plaque assay dilution series
Plaque assay dilution series (Photo credit: Wikipedia)

Since a virus cannot move it only infects cells directly adjacent to those it has already infected. When a virus is diluted down to a very low concentration and added to a single layer of cells you will see first a single cell get infected, begin to produce virus, and die, then the cells around it in an outward fashion forming a circular focus of dead cells known as a plaque. By counting the number of these plaques formed on a layer of cells and accounting for the dilution it is possible to determine the amount of virus in a sample with a good degree of precision. This method is commonly used to determine the titer (plaque forming units/volume) of a sample for use in further experiments. While viruses cannot move through the environment on their own, a fact exploited by researchers in the laboratory, they have developed strategies that are intimately tied to their life-cycles in order to get from one host to the next.

1.        McKerrow, J. & Salter, J. Invasion of skin by Schistosoma cercariae. Trends in Parasitology 18, 193-195 (2002).

This post appeared in original form on on April 28th, 2012.

[Featured image from Wikimedia Commons used under Creative Commons License.]

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