The problem in your image is asking to calculate the Original Cell Density (OCD) in a 1 mL culture of bacteria, using the unit of Colony-Forming Units per milliliter (CFU/mL). The formula provided is:

$\text{OCD} = \frac{\text{CFU}}{\text{V} \times \text{D}}$

Where:

• CFU is the number of Colony-Forming Units,
• $V$ is the volume plated (mL),
• $D$ is the dilution factor.

Data from the image:

1. 350 colonies:

   • CFU = 350
   • Dilution factor (D) = $10^{-4}$
   • Volume plated (V) = 0.1 mL $\text{OCD} = \frac{350}{0.1 \times 10^{-4}} = 3.5 \times 10^6 \text{ CFU/mL}$

2. 140 colonies:

   • CFU = 140
   • Dilution factor (D) = $10^{-2}$
   • Volume plated (V) = 0.1 mL $\text{OCD} = \frac{140}{0.1 \times 10^{-2}} = 1.4 \times 10^5 \text{ CFU/mL}$

3. 5 colonies:

   • CFU = 5
   • Dilution factor (D) = $10^{-3}$
   • Volume plated (V) = 0.1 mL $\text{OCD} = \frac{5}{0.1 \times 10^{-3}} = 5 \times 10^4 \text{ CFU/mL}$

These results match the calculations written on your sheet, confirming that everything looks accurate.

Would you like further details or have any questions?

Related Questions:

1. How does changing the dilution factor affect the Original Cell Density?
2. What happens to the calculated OCD if the volume plated changes?
3. Can this method be used for other types of cells, and why?
4. Why is CFU/mL used instead of other units like CFU/g?
5. What are some potential sources of error when calculating OCD?

Tip:

Always ensure accurate measurements for the volume plated and dilutions to avoid significant errors in OCD calculations.