What MOI means
The multiplicity of infection (MOI) is the number of infectious virus particles added per cell. An MOI of 5 on 200,000 cells means adding 1,000,000 infectious units.
The formula
Volume of virus (mL) = MOI × number of cells ÷ titer (infectious units per mL)
For 200,000 cells at MOI 5 with a titer of 1 × 108 per mL: 5 × 200,000 ÷ 108 = 0.01 mL = 10 µL.
Count the cells on the day of infection, since they keep dividing after seeding, and use a titer measured the way the virus infects (PFU for plaque-forming viruses, transducing units for lentivirus). Physical titers such as genome copies or p24 count non-infectious particles too and overstate the infectious dose.
How many cells get infected
Virus particles land on cells at random, which follows the Poisson distribution. The share of cells infected at least once is 1 − e−MOI:
- MOI 0.1: about 10% of cells
- MOI 1: about 63%
- MOI 3: about 95%
- MOI 5: about 99%
So an MOI of 1 does not infect every cell; reaching nearly all of them takes an MOI of 3 to 5.
Which MOI to use
- To infect nearly every cell, such as for a one-step growth curve: MOI 3 to 10.
- For pooled library screens, where each cell should carry one construct: about 0.3, so only about a quarter of cells are infected but about 86% of those receive a single copy.
- For multi-step growth curves, where the virus spreads through the culture: 0.01 to 0.1.
Calculators
The MOI Calculator gives the volume of virus for an MOI, or the MOI for a volume, with the expected share of infected cells. To count your cells first, use the Hemocytometer Calculator.