Are all virus particles infectious?

particle to pfu ratioChris Upton, a contributor to the virology toolbox, has raised an important point about multiplicity of infection:

Perhaps this is a place to bring up particle to pfu ratio? The above is great for when talking about phage, for example, when the ratio approaches 1. But with something like polio when it can be very high (>1000 ??), then it’s not that all cells don’t receive “a particle” at MOI=1 – but that they don’t get an “infectious dose”. Not sure how to say it better – enough to initiate an infection.

So why does polio require 1000 virions to make an infectious dose? I don’t buy the idea that most of the particles are not “viable”.

If we take the titer of a virus preparation (in plaque forming-units per milliliter) and divide it by into the number of virus particles in the sample, we obtain a number known as the particle-to-PFU ratio. It is a measure of the fraction of virus particles in a given sample that can complete an infectious cycle. For many bacteriophages, the particle-to-PFU ratio approaches 1, which is the lowest value that can be obtained. A value of 1 means that every virus particle in the sample is able to form a plaque.

For animal viruses, the particle-to-pfu ratio is often much higher, from 1 to 10,000 (the image shows values for different animal viruses – click to enlarge). These high values complicate the study of animal viruses. When the particle-to-pfu ratio is high, one can never be certain that properties measured in infected cells are those of the infectious or the non-infectious viral particles.

The linear nature of the dose-response curve indicates that a single virion is capable of initiating an infection. However, the high particle-to-pfu ratio of many viruses shows that not all virions are successful. A high particle-to-pfu ratio is sometimes caused by the presence of noninfectious particles with genomes that harbor lethal mutations or that have been damaged during growth or purification. Another explanation is that although all viruses in a preparation are in fact capable of initiating infection, not all of them succeed because of the complexity of the infectious cycle. Failure at any one step in the cycle prevents completion.

A high particle-to-pfu ratio does not indicate that most particles are defective, but that they failed to complete the infection.

How many viruses are needed to form a plaque?

The plaque assay is an essential tool for determining virus titers. The concept is simple: virus infection is restricted to neighboring cells by a semisolid overlay. By counting the number of plaques, the virus titer can be calculated in PFU per ml. A key question is: how many viruses are needed to form a single plaque?

For most animal viruses, one infectious particle is sufficient to initiate infection. This conclusion can be reached by studying the relationship between the number of infectious virus particles and the plaque count. A linear relationship means that one infectious particle can form a plaque. In this case the virus is said to infect cells with one-hit kinetics. This concept is illustrated below. In this figure, the number of plaques produced by a virus with one-hit kinetics or two-hit kinetics is plotted versus the relative concentration of the virus.


There are some examples of viruses with two-hit kinetics: in other words, two different types of viral particles must infect a cell to initiate the infectious cycle. Examples include the genomes of some (+) strand RNA viruses of plants, which consists of two RNA molecules that are packaged in different particles. The dose-response curve of such viruses is parabolic rather than linear.

When a single virus particle can form a plaque, the viral progeny within the plaque are clones. Virus stocks prepared from a single plaque are called plaque purified virus stocks. To prepare such virus stocks, the tip of a small pipette is inserted into the agar overlay above the plaque. The plug of agar is removed and placed in buffer. The viruses within the agar plug move into the buffer, which can then be used to infect cultured cells. To ensure purity, this process is usually repeated at least one more time. Plaque purification is used extensively in virology to establish clonal virus stocks. The ability to prepare clonal virus stocks was an essential development that permitted genetic analysis of viruses.

Detecting viruses: the plaque assay

One of the most important procedures in virology is measuring the virus titer – the concentration of viruses in a sample. A widely used approach for determining the quantity of infectious virus is the plaque assay. This technique was first developed to calculate the titers of bacteriophage stocks. Renato Dulbecco modified this procedure in 1952 for use in animal virology, and it has since been used for reliable determination of the titers of many different viruses.

polio-plaquesTo perform a plaque assay, 10-fold dilutions of a virus stock are prepared, and 0.1 ml aliquots are inoculated onto susceptible cell monolayers. After an incubation period, to allow virus to attach to cells, the monolayers are covered with a nutrient medium containing a substance, usually agar, that causes the formation of a gel. When the plates are incubated, the original infected cells release viral progeny. The spread of the new viruses is restricted to neighboring cells by the gel. Consequently, each infectious particle produces a circular zone of infected cells called a plaque. Eventually the plaque becomes large enough to be visible to the naked eye. Dyes that stain living cells are often used to enhance the contrast between the living cells and the plaques. Only viruses that cause visible damage of cells can be assayed in this way. An example of plaques formed by poliovirus on a monolayer of HeLa cells is shown at left. In this image, the cells have been stained with crystal violet, and the plaques are readily visible where the cells have been destroyed by viral infection.

The titer of a virus stock can be calculated in plaque-forming units (PFU) per milliliter. To determine the virus titer, the plaques are counted. To minimize error, only plates containing between 10 and 100 plaques are counted, depending on the size of the cell culture plate that is used. Statistical principles dictate that when 100 plaques are counted, the sample titer will vary by plus or minus 10%. Each dilution is plated in duplicate to enhance accuracy. In the example shown below, there are 17 plaques on the plate made from the 10-6 dilution. The titer of the virus stock is therefore 1.7 x 108 PFU/ml.


Next we’ll consider how the plaque assay can be used to prepare clonal virus stocks, a step that is essential for studying viral genetics.

Dulbecco, R., & Vogt, M. (1953). Some problems of animal virology as studied by the plaque technique. Cold Spring Harbor Symp. Quant. Biol., 18, 273-279