Question

A pre-med student taking BIOL 350 makes 3 serial dilutions of her bacteriophage stock in order to determine the phage titer. She dilutes the stock by pipetting 0.1 mL of phage stock into 9.9 mL of phage dilution buffer, mixes well, and pipets 0.1 mL of this dilution into a second tube of 9.9 mL dilution buffer, repeats this two more times, then finally pipets 0.1 mL of the fourth dilution into a fifth tube of 0.9 mL dilution buffer. From the fifth phage dilution tube, she pipets 0.15 mL to a plate with susceptible cells in melted top agar. After overnight incubation, she counts 256 plaques, aka PFUs or Plaque-Forming Units. What is the titer of her bacteriophage stock in PFU/mL? Answer in decimal notation.

Answer 1

PFU/mL= 1.71×10⁸

Step-by-step explanation:

By counting the number of plaques, one can calculate the titer of the virus sample (often expressed as “plaque-forming units” per milliliter).

We can use the following equation:

PFU/mL = nº of plaques ÷ (d × v)

Where d: dilution

v: volume of diluted virus added to the plate

PFU/mL= 256 ÷ (10⁻⁶ × 0.15)

(In the fourth tube she pipets 0.1 mL into 0.9 mL, instead of 9.9 mL, so she made two dilutions at one, hence the dilution=10⁻⁶).

PFU/mL= 1.71×10⁸