A pre-med student taking BIOL 350 makes 3 serial dilutions of her bacteriophage

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 one more time, then finally pipets 0.1 mL of the third dilution into a fourth tube of 0.9 mL dilution buffer. From the fourth phage dilution tube, she pipets 0.2 mL to a plate with susceptible cells in melted top agar. After overnight incubation, she counts 428 plaques, aka PFUs or Plaque-Forming Units. What is the titer of her bacteriophage stock in PFU/mL? Answer in decimal notation.

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Explanation / Answer

titer of bacteriophage stock pfu/ml = number of plaques / d* v where d is dilution and v is volume of virus added.

number of plaques observed = 428

volume of virus added = 0.2

pfu/ml = 428 / 10*0.2 = 214 pfu/ml

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