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In microbiology, the multiplicity of infection or MOI is the ratio of agents (e.g. phage or more generally virus, bacteria) to infection targets (e.g. cell). For example, when referring to a group of cells inoculated with virus particles, the multiplicity of infection or MOI is the ratio of the number of virus particles to the number of target cells present in a defined space.
Contents
Interpretation
The actual number of viruses or bacteria that will enter any given cell is a statistical process: some cells may absorb more than one infectious agent while others may not absorb any. The probability that a cell will absorb
where
In fact the infectivity of the virus or bacteria in question will alter this relationship. One way around this is to use a functional definition of infectious particles rather than a strict count, such as a plaque forming unit for viruses.
For example, when an MOI of 1 (1 viral particle per cell) is used to infect a population of cells, the probability that a cell will not get infected is
The average percentage of cells that will become infected as a result of inoculation with a given MOI can be obtained by realizing that it is simply
which is approximately equal to
Examples
As the MOI increases, the percentages of cells infected with at least one viral particle also increases.
Fields' virology, Part 1 By Bernard N. Fields,David Mahan Knipe,Peter M. Howley,Diane E. Griffin