GATE 2021 BT – Question 50
0.1 mL phage stock at $4\times10^9$ phages/mL is added to 0.5 mL E. coli at $2\times10^8$ cells/mL. Multiplicity of infection is _______.
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Correct answer: 4
Explanation
The **multiplicity of infection (MOI)** is the ratio of the number of phage particles to the number of bacterial cells:
$$MOI=\frac{\text{phages added}}{\text{bacteria}} .$$
**Phages:** $0.1\text{ mL}\times4\times10^9\text{ mL}^{-1}=4\times10^8$.
**Bacteria:** $0.5\text{ mL}\times2\times10^8\text{ mL}^{-1}=1\times10^8$.
$$MOI=\frac{4\times10^8}{1\times10^8}=\mathbf{4}.$$