Multiplicity of infection

Interpretation

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 virus particles or bacteria when inoculated with an MOI of m can be calculated for a given population using a Poisson distribution. This application of Poisson's distribution was applied and described by Ellis and Delbrück.

: P(n) = \frac{m^n \cdot e^{-m}}{n!}

where m is the multiplicity of infection or MOI, n is the number of infectious agents that enter the infection target, and P(n) is the probability that an infection target (a cell) will get infected by n infectious agents.

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 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%, and the probability that it be infected by a single particle is P(1) = 36.79%, by two particles is P(2)=18.39%, by three particles is P(3) = 6.13%, and so on.

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(n0) = 1 - P(0). Hence, the average fraction of cells that will become infected following an inoculation with an MOI of m is given by:

: P(n0) = 1 - P(n=0) = 1 - \frac{m^0 \cdot e^{-m}}{0!} = 1 - e^{-m}

which is approximately equal to m for small values of m \ll 1.

Example

As the MOI increases, the percentages of cells infected with at least one viral particle (n0) also increases.

References

References

1. (2013-01-01). "Multiplicity of Infection". [[Academic Press]].
2. (Jan 20, 1939). "The Growth of Bacteriophage". [[The Journal of General Physiology]].
3. "Plaque forming unit".
4. (2007). "Fields virology: Part 1". Wolters Kluwer Health/Lippincott Williams & Wilkins.