There are two important definitions relating to infectious virus particles or PFUs. The particle to PFU ratio measures just that: the proportion of total number of virus particles to infectious particles of virus. To obtain the ratio, one must count the total virus particles and do an assay for biologically functional ones.
Some types of viruses (bacteriophages, and under very special circumstances, poliovirus, for example) have particle to PFU ratios approaching 1. Careful preparations of viruses such as adenovirus and HSV can have ratios of less than 10, but the best ratios for influenza A virus are on the order of 103. This high particle to PFU ratio is unusual but is inherent in the way that flu virus virions are formed. Particle to PFU ratios can vary depending on the specifics of the particular infection and virus, and each virus type has a characteristic optimum value that tells something about the efficiency of encapsidation and release of infectious virus from infected cells.
A second quantitative measure of conditions of virus infection is the multiplicity of infection (MOI), which is simply the average number of PFUs per cell utilized in the original infection. An MOI of 1 means 1 PFU per cell, so if 106 cells were infected at an MOI of 1, one would need to add 106 PFUs of virus. It is important to note that an MOI can vary from zero to a very high number, depending on the concentration of virus in the original stock, the type of experimental problem being studied, and so on. MOI measures an average value; statistical analysis that demonstrates the number of PFUs interacting with one individual cell can vary over a wide range when a culture is infected at MOIs greater than 0.1 or so.
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