Which of the following formulas is used to calculate the original cell density (OCD) in a sample after performing a serial dilution and plate count?

• A. OCD = Number of colonies / (Dilution factor × Volume plated in mL)
• B. OCD = Number of colonies × Dilution factor × Volume plated in mL
• C. OCD = Number of colonies / (Volume plated in mL)
• D. OCD = Dilution factor / (Number of colonies × Volume plated in mL)

Answer: (A)

Verified step by step guidance

1. Understand that the original cell density (OCD) represents the concentration of cells in the original sample before dilution.
2. Recognize that after performing serial dilutions and plating a known volume, the number of colonies counted corresponds to the number of viable cells in that plated volume at that dilution.
3. Recall the formula to calculate OCD, which adjusts the counted colonies by accounting for both the dilution factor and the volume plated, to estimate the concentration in the original sample.
4. Express the formula as: \(\text{OCD} = \frac{\text{Number of colonies}}{\text{Dilution factor} \times \text{Volume plated in mL}}\).
5. Use this formula to calculate the original cell density by dividing the colony count by the product of the dilution factor and the plated volume.