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MOI Calculator (Multiplicity of Infection)

Calculate the volume of virus (or bacteria) to add for a target multiplicity of infection, or the MOI from the volume you added, with the Poisson fraction of cells infected.

Formula

V=MOI×NcellstiterV = \dfrac{\mathrm{MOI}\times N_{\text{cells}}}{\text{titer}}
MOI=titer×VNcells\mathrm{MOI} = \dfrac{\text{titer}\times V}{N_{\text{cells}}}
P(k)=e−MOI MOIkk!P(k) = \dfrac{e^{-\mathrm{MOI}}\,\mathrm{MOI}^{k}}{k!}
MOI\mathrm{MOI}
multiplicity of infection: infectious units per cell
titer\text{titer}
infectious units per mL (pfu, TU or CFU)

How it works

The multiplicity of infection is the ratio of infectious units to cells. To infect a dish, multiply the number of cells by the MOI to get the units needed, and divide by the titer of the stock to get the volume to add.

The MOI is an average. Because virions land on cells at random, the number each cell receives follows a Poisson distribution, which the page tabulates. At an MOI of 1, about 37% of cells receive no virus and 26% receive two or more; to infect nearly every cell, an MOI of 5 or more is needed, which also gives most cells several particles. A low MOI, near 0.1, is used when single infections are wanted.

Worked example

10⁶ cells, a stock of 10⁸ pfu/mL and a target MOI of 5.

  1. Infectious units needed = 5 × 10⁶.
  2. Volume = 5 × 10⁶ / 10⁸ per mL = 0.05 mL.

Add 50 µL of stock. At MOI 5, 99.3% of cells receive at least one unit, but 87.5% receive three or more.

These are the values the calculator opens with, so you can check its output against this example.

Assumptions

  • The titer is of infectious units on cells like yours, not of physical particles.
  • Virions are randomly distributed over cells, and adsorb equally.
  • The number of cells is that at the time of infection.

Common mistakes

  • Using the physical particle count instead of the infectious titer.
  • Counting the cells seeded rather than the cells present at infection.
  • Reading an MOI of 1 as meaning that every cell is infected once.

Related tools

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Related equipment

Service documentation, failure modes and parts for the instruments this calculation is used with.