Q.50 A 0.1 mL aliquot of a bacteriophage stock having a concentration of
4 × 10⁹ phages mL⁻¹ is added to 0.5 mL of E. coli culture having a
concentration of 2 × 10⁸ cells mL⁻¹. The multiplicity of infection is _________.

Multiplicity of infection (MOI) measures the ratio of bacteriophages to bacterial cells in virology experiments. In this case, adding 0.1 mL of 4 × 10⁹ phages mL⁻¹ stock to 0.5 mL of 2 × 10⁸ E. coli mL⁻¹ culture yields an MOI of 4.

MOI Formula

Multiplicity of infection equals total infecting phages divided by total target cells: MOI = (phage particles) / (bacterial cells). This dimensionless ratio guides infection outcomes, like single-hit kinetics at MOI <1 or multi-hit at MOI >1.

Step-by-Step Calculation

First, compute phages added: 0.1 mL × 4 × 10⁹ phages mL⁻¹ = 4 × 10⁸ phages.

Next, compute E. coli cells: 0.5 mL × 2 × 10⁸ cells mL⁻¹ = 1 × 10⁸ cells.

Finally, MOI = (4 × 10⁸) / (1 × 10⁸) = 4.

Common Errors Explained

• Using concentrations directly without volumes gives 20, ignoring dilution.
• Forgetting mL-to-L conversion or assuming 1 mL total volume leads to 2 or 8.
• MOI ignores adsorption efficiency; actual infection may vary.

Common Calculation Errors