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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 MOI is the ratio of the number of virus particles to the number of target cells present in a defined space.^[1]

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.^[2]

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.^[3]

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

P(n>0)=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 $m\ll 1$ .

Examples

Percentage of cells infected based on MOI.

As the MOI increases, the percentages of cells infected with at least one viral particle also increases.^[4]

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%

References

^ Abedon, S. T.; Bartom, E. (2013-01-01), "Multiplicity of Infection", in Maloy, Stanley; Hughes, Kelly (eds.), Brenner's Encyclopedia of Genetics (Second Edition), San Diego: Academic Press, pp. 509–510, ISBN 978-0-08-096156-9, retrieved 2022-03-09
^ Ellis, Emory; Delbruck, Max (Jan 20, 1939). "The Growth of Bacteriophage". The Journal of General Physiology. 22 (3): 365–384. doi:10.1085/jgp.22.3.365. PMC 2141994. PMID 19873108.
^ "Plaque forming unit". Science Direct.
^ Fields BN, Knipe DM, Howley PM (2007). Fields virology: Part 1. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. ISBN 9780781760607. OCLC 71812790.

[1] Abedon, S. T.; Bartom, E. (2013-01-01), "Multiplicity of Infection", in Maloy, Stanley; Hughes, Kelly (eds.), Brenner's Encyclopedia of Genetics (Second Edition), San Diego: Academic Press, pp. 509–510, ISBN 978-0-08-096156-9, retrieved 2022-03-09

[2] Ellis, Emory; Delbruck, Max (Jan 20, 1939). "The Growth of Bacteriophage". The Journal of General Physiology. 22 (3): 365–384. doi:10.1085/jgp.22.3.365. PMC 2141994. PMID 19873108.

[3] "Plaque forming unit". Science Direct.

[4] Fields BN, Knipe DM, Howley PM (2007). Fields virology: Part 1. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. ISBN 9780781760607. OCLC 71812790.

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Interpretation

Examples

See also

References