How to Calculate CFU per ml of Bacterial Sample? in 3 Steps || cfu/ml in Microbiology

Summary

In this video, we walk through the process of calculating Colony Forming Units (CFU) per mL of an original stock solution. The procedure is broken into three simple steps: selecting a plate with countable colonies (between 30-300), determining the Total Dilution Factor (TDF) from serial dilution, and substituting these values into a formula to get CFU per mL. By following this method, viewers can accurately determine the concentration of bacterial colonies in a sample, with special attention given to volume plated and dilution factors for reliable results.

Takeaways

• 😀 Choose a plate with 30 to 300 colonies for accurate counting in microbiological experiments.
• 😀 Plates with fewer than 30 colonies or more than 300 colonies are prone to statistical errors.
• 😀 The total dilution factor is the product of dilution factors from each step of serial dilution.
• 😀 Dilution factor is calculated as the final volume divided by the initial volume (e.g., 10 mL / 1 mL).
• 😀 In serial dilution, the dilution factor increases by a power of 10 in each subsequent test tube.
• 😀 The selected plate for counting should correspond to the appropriate test tube's dilution factor.
• 😀 The total dilution factor from the chosen test tube helps in calculating the CFU per mL of the original stock.
• 😀 Volume plated in the calculation must be considered (e.g., 0.1 mL instead of 1 mL).
• 😀 The formula to calculate CFUs per mL is: CFU per mL = (Number of colonies * Total dilution factor) / Volume plated.
• 😀 Accurate CFU per mL calculation is crucial for understanding the concentration of bacteria in the original stock solution.

Q & A

What is the equation used to calculate CFU per mL of the original stock?

The equation used to calculate CFU per mL is: CFU/mL = (Number of Colonies × Total Dilution Factor) ÷ Volume of Culture Plated (in mL).

Why is it important to select a plate with between 30 to 300 colonies?

It is important because this range ensures statistically reliable results. Fewer than 30 colonies make the sample size too small, while more than 300 can cause overcrowding, leading to higher chances of error.

How do you calculate the dilution factor for a single test tube?

The dilution factor for a test tube is calculated using the formula: Dilution Factor = Final Volume ÷ Initial Volume. For example, if the final volume is 10 mL and the initial volume is 1 mL, the dilution factor would be 10.

What happens to the dilution factor as you progress through the test tubes in a serial dilution?

As you progress through the test tubes in a serial dilution, the dilution factor increases exponentially. For example, if each subsequent tube dilutes the sample by a factor of 10, the total dilution factor will be the product of the dilution factors of each tube.

How is the total dilution factor calculated in serial dilution?

The total dilution factor is calculated by multiplying the dilution factors of all the test tubes used in the serial dilution. For example, if Test Tube A has a dilution factor of 10, Test Tube B has a dilution factor of 10², and Test Tube C has a dilution factor of 10³, the total dilution factor is 10 × 10² × 10³.

What volume of culture was plated from the stock solution, and why is this important?

A volume of 0.1 mL was plated from the stock solution. This is important because the volume plated directly affects the final calculation of CFU per mL. If a different volume had been plated, the dilution and final CFU result would change.

What is the significance of the dilution factor of 10⁵ in this example?

The dilution factor of 10⁵ is significant because it corresponds to the culture that was used in the calculation based on the plate with 63 colonies. This dilution factor reflects how much the original stock solution was diluted before plating.

How does the equation change if a 1 mL volume was plated instead of 0.1 mL?

If a 1 mL volume was plated, the equation would simplify to CFU/mL = Number of Colonies × Total Dilution Factor. In this case, the dilution factor would be applied without needing to divide by 0.1 mL.

What was the final CFU/mL calculated in this example, and how was it expressed scientifically?

The final CFU/mL calculated was 63 × 10⁶, which is scientifically expressed as 6.3 × 10⁷ CFU/mL.

Why is it crucial to use the correct dilution factor when calculating CFU/mL?

Using the correct dilution factor is crucial because it ensures the accuracy of the CFU/mL calculation. An incorrect dilution factor would result in a significant error in determining the number of colony-forming units in the original stock.