Question

In: Biology

1a. If a 10^-2 dilution of a culture yields 150 colonies when 0.5 ml is spread...

1a. If a 10^-2 dilution of a culture yields 150 colonies when 0.5 ml is spread on a plate, how many colonies will grow on a plate that receives 0.1 ml of the same dilution?

1b. How many CFU per ml are present in a sample if 0.2 ml of a 10^-5 dilution grew 220 colonies?

1c. How many colonies do you expect when 0.4 ml of a 10^-7 dilution of a culture containing 10^9 CFU per ml is plated onto nutrient media?

Expert Solution

1a. Given the number of colonies obtained from a 0.5 mL of 10^-2 dilution of a culture = 150 colonies

We need to determine the number of colonies if 0.1 mL of the same dilution is plated.

The dilution being the same, we are taking 1 / 5th (0.5 / 0.1 = 5) the original volume for plating. This means the number of colonies that appear on the plate would reduce in size by 5 times the original number.

150 / 5 = 30.

The number of colonies with 0.1 mL of 10^-2 dilution = 30.

1b. Given the number of colonies = 220

Dilution factor = 10⁵ (dilution factor is the inverse of dilution used for plating).

Volume plated = 0.2 mL.

CFU / mL = (No. of colonies * Dilution factor) / volume plated

= 220 * 10⁵ / 0.2 = 1100 * 10⁵ = 1.1 * 10⁸.

Therefore, the concentration of the original culture is of the sample is 1.1 * 10⁸ CFU / mL.

1c. Given the concentration of a culture = 10⁹ CFU / mL

Volume plated = 0.4 mL

Dilution factor = 10⁷.

We are asked to determine the number of colonies obtained. this can be found out using the formula for CFU / mL:

CFU / mL = No. of colonies * Dilution factor / Volume plated.

the above equation can be written as:

No. of colonies = CFU / mL* volume plated / Dilution factor

= 10⁹ * 0.4 / 10⁷ = 10⁹ * 0.4 * 10^-7 = 10² * 0.4 = 100 * 0.4 = 40

Therefore the number of colonies obtained by plating 0.4 mL of 10^-7 dilution = 40 colonies.

gladiator answered 1 hour ago

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