microbiology Microbiology () is the scientific study of microorganisms, those being unicellular (single cell), multicellular (cell colony), or acellular (lacking cells). Microbiology encompasses numerous sub-disciplines including virology, bacteriology, ...

, the multiplicity of infection or MOI is the

ratio In mathematics, a ratio shows how many times one number contains another. For example, if there are eight oranges and six lemons in a bowl of fruit, then the ratio of oranges to lemons is eight to six (that is, 8:6, which is equivalent to the ...

of agents (e.g.

phage A bacteriophage (), also known informally as a ''phage'' (), is a duplodnaviria virus that infects and replicates within bacteria and archaea. The term was derived from "bacteria" and the Greek φαγεῖν ('), meaning "to devour". Bacter ...

or more generally

virus A virus is a submicroscopic infectious agent that replicates only inside the living cells of an organism. Viruses infect all life forms, from animals and plants to microorganisms, including bacteria and archaea. Since Dmitri Ivanovsk ...

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...

) 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.

Interpretation

The actual number of viruses or bacteria that will enter any given cell is a

stochastic process In probability theory and related fields, a stochastic () or random process is a mathematical object usually defined as a family of random variables. Stochastic processes are widely used as mathematical models of systems and phenomena that ap ...

: 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 Probability is the branch of mathematics concerning numerical descriptions of how likely an event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and 1, where, roughly speaking, ...

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 In probability theory and statistics, the Poisson distribution is a discrete probability distribution that expresses the probability of a given number of events occurring in a fixed interval of time or space if these events occur with a known co ...

. This application of Poisson's distribution was applied and described by Ellis and Delbrück. :

P(n) = \frac

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 A plaque-forming unit (PFU) is a measure used in virology to describe the number of virus particles capable of forming plaques per unit volume. It is a proxy measurement rather than a measurement of the absolute quantity of particles: viral pa ...

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(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 = 1 - e^

which is approximately equal to

m

for small values of

m \ll 1

Examples

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

References

{{DEFAULTSORT:Multiplicity Of Infection Virology Bacteriology Bacteriophages

Interpretation

Examples

See also

References