Q.84 An actively growing culture (20 ml) of E. coli (1 × 105 per ml) was mixed with a total of 100 T4 phage particles,
grown further for 40 min and mixed with a few drops of chloroform. Under the conditions used, the generation time of E. coli is 30 min,
the infection cycle of phage T4 is 20 min, and the burst size is 100. Assuming that each infection was a successful one,
how many plaque forming units would you expect at the end of the experiment?
E Coli T4 Phage Plaque Forming Units Calculation MCQ Solved: 20 ml Culture 1×105/ml 100 Phages 40 Min
In this T4 phage infection experiment with E. coli, 100 phages infect 100 cells, lyse after 20 min releasing 10,000 progeny, and with no further lysis by 40 min, total plaque forming units (PFU) expected is 104. The correct answer is (A).
Correct Answer
(A) 104
This calculation follows standard T4 phage one-step growth experiment protocol where only the first burst contributes to PFU at 40 minutes.
Step-by-Step Calculation
Initial setup: 20 ml culture at 1 × 105 cells/ml gives 2 × 106 E. coli cells. Add 100 T4 phages; assuming multiplicity of infection (MOI) ≪1 and all successful, 100 cells infected.
- 0-20 min: Infected cells adsorb phages instantly, eclipse/intracellular growth. Uninfected grow one generation (30 min gen time).
- 20 min: 100 infected lyse, burst size 100 → 100 × 100 = 10,000 progeny phages released (first burst).
- 20-40 min: Progeny adsorb to uninfected cells but no second lysis. Chloroform added lyses all cells, releases only first progeny as PFU.
Phage Infection Timeline
| Time Point | Infected Cells | Free Phages | Uninfected Cells (approx.) |
|---|---|---|---|
| 0 min | 100 | 0 | 2 × 106 |
| 20 min | Lyse | 104 | 4 × 106 |
| 40 min | 104 new | 0 (adsorbed) | 8 × 106 |
Introduction to T4 Phage One-Step Growth Experiment
E coli T4 phage plaque forming units calculation tests phage dynamics: infection cycle 20 min, burst size 100, generation time 30 min in 40 min experiment with 20 ml culture at 1 × 105/ml and 100 phages. Key for virology, biotech exams understanding lysis, progeny release.
