Dustin Edwards

MOI Calculator

MOI calculator

1e9 or 1 x 109.

Lab default: Phage Isolation and Purification Protocol: full plate titer.

Counted, not read off an OD: the site gives no OD-to-cells factor for the lab's hosts. 1e8 is only the worked example's number.

Lab default: Phage Isolation and Purification Protocol: full plate titer.

MOI: 0.4 pfu per cell

MOI

  1. pfu added: 1 x 109 pfu/ml x 10 µl / 1,000 µl/ml = 1 x 107 pfu.
  2. Cells: 1 x 108 cells/ml x 250 µl / 1,000 µl/ml = 2.5 x 107 cells.
  3. MOI: 1 x 107 pfu / 2.5 x 107 cells = 0.4.
  4. Cells that get at least one phage: 1 - exp(-0.4) = 0.33, about 33%.

The last step assumes phages meet cells at random and every pfu adsorbs. Slow or incomplete adsorption infects fewer cells than it says.

The multiplicity of infection (MOI) is the number of phages added per host cell. It sets how many cells are infected in the first round: at a low MOI most cells get no phage, and at a high MOI nearly every cell gets at least one.

Formula#

pfu added = titer (pfu/ml) x µl added / 1,000 µl/ml
cells = cells/ml x µl of culture / 1,000 µl/ml
MOI = pfu added / cells
share of cells infected = 1 - exp(-MOI)

The last line assumes phages meet cells at random (a Poisson distribution) and that every pfu adsorbs, so it is the most the MOI can infect. At an MOI of 1, about 63% of cells get at least one phage and about 37% get none.

Worked example#

These numbers are illustrative, chosen to make the arithmetic easy to follow: 10 µl of a lysate at 1 x 10^9 pfu/ml added to 250 µl of a host culture at 1 x 10^8 cells/ml.

  1. pfu added: 1 x 10^9 pfu/ml x 10 µl / 1,000 µl/ml = 1 x 10^7 pfu.
  2. Cells: 1 x 10^8 cells/ml x 250 µl / 1,000 µl/ml = 2.5 x 10^7 cells.
  3. MOI: 1 x 10^7 pfu / 2.5 x 10^7 cells = 0.4.
  4. Cells that get at least one phage: 1 - exp(-0.4) = 0.33, about 33%.

The MOI is 0.4 pfu per cell.

Counting the cells#

The MOI is only as good as the cell count. Count the host culture, for example by plating a dilution of it and counting colonies. This site gives no conversion from an optical density reading to cells per ml for the lab's hosts, so the calculator takes cells per ml and does not guess one from an OD. The 1 x 10^8 cells/ml in the example is not a measured value for any host.

Where the numbers come from#

  • 10 µl of phage with 250 µl of host. The lab's full plate titer plates 10 µl of each dilution with 250 µl of host: full plate titer. The calculator opens with these volumes; change them to match your own experiment.
  • The titer comes from the titer calculator.

The other calculators are the titer calculator, the serial dilution planner, the webbed plate calculator, the efficiency of plating calculator and the lysate volume planner.