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 MOI is the ratio of the number of virus particles to the number of target cells present in a defined space.
The actual number of viruses or bacteria that will enter any given cell is a stochastic process: some cells may absorb more than one infectious agent, while others may not absorb any. Before determining the multiplicity of infection, it's absolutely necessary to have a well-isolated agent, as crude agents may not produce reliable and reproducible results. The probability that a cell will absorb
n
m
P(n)=
| mn ⋅ e-m | |
| n! |
where
m
n
P(n)
n
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.[2]
For example, when an MOI of 1 (1 infectious viral particle per cell) is used to infect a population of cells, the probability that a cell will not get infected is
P(0)=36.79\%
P(1)=36.79\%
P(2)=18.39\%
P(3)=6.13\%
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
P(n>0)=1-P(0)
m
P(n>0)=1-P(n=0)=1-
| m0 ⋅ e-m | |
| 0! |
=1-e-m
which is approximately equal to
m
m\ll1
As the MOI increases, the percentages of cells infected with at least one viral particle also increases.[3]
| MOI | % Infected | |
|---|---|---|
| 1.0 | 63.2% | |
| 2.0 | 86.5% | |
| 3.0 | 95.0% | |
| 4.0 | 98.2% | |
| 5.0 | 99.3% | |
| 6.0 | 99.8% | |
| 7.0 | 99.9% | |
| 8.0 | ~100.0% |