Question

I have done these problems but am unsure if the answers I got were correct, Please show all steps so I can fully understand!

1.)

A 0.1 dilution is performed on a culture of bacteria in order to perform viable plate counts. From the dilution, *0.1 mL* of solution is plated on solid media, and 154 colony forming units grow on the plate. How many bacteria are in a single mL of the original culture? Express your answer to two decimal places using exponential notation.

• Since only 0.1 mL is put on the plate, this counts as an extra dilution!!!

2.) There are so many microbes in a single mL of culture, it is very difficult to perform one dilution to produce countable cells. Microbiologists need to perform a dilution series, where multiple dilutions are performed in sequence to arrive at the correct dilution.

Dilutions are cumulative. Multiply the series of dilutions together to find the final dilution value.

If 2 serial dilutions are performed, each with a dilution of 0.01, what is the cumulative dilution?

Answer 1

1. 154 colonies are present in 0.1 mL solution.

So, 1 mL will have 154 ÷ 0.1 = 1540 colonies.

But 1540 colonies are present in 0.1 dilution.

So, # colonies in orginal solution (1x concentration) will be 1540 ÷ 0.1 = 15400

OR

You can directly use the formula -

Original cell concentration in CFU per mL = #colonies / (dilution × volume plated) = 154 / (0.1 × 0.1) = 15400

2. Dilution 1 = 0.01

Dilution 2 = 0.01

Final dilution (multiply all dilutions) = 0.01 × 0.01 = 0.0001 = 10^-4 = 1/10000

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