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.

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. 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, and the probability that it be infected by a single particle is , by two particles is , by three particles is , 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. Hence, the average fraction of cells that will become infected following an inoculation with an MOI of m is given by: 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.