Question

If you estimate that an overnight culture has 5.28x10^7 viable colonies per mL, how would you confirm this? Please draw ALL dilutions and write what dilution each tube and plate represents and how many bacteria you expect per mL in each tube. Write down how many bacteria will be on the final plate. Making sure that the number falls within the expected range for accurate counts.

Answer 1

We can estimate the number of cells/Colony forming units by serial dlutions and plating

The initial estimate is 5.28 10^7 colonies. we will consider the colonies mentioned here as individual cells

This means that this culture contains enough cells to form this many colonies on a plate

Since that is too many we will then serially dilute the culture until it gives us a lower number of colony forming units

The first dilution is take 0.1 ml of overnight culture and add to 9.9 ml broth

So that is a 100 times dilution divide the initial cell no by 100

5.28 10^5

Now repeat the above step for another dilution

Serial dilution 2 contains

5.2810^3

repeat again

Serial dilution 3 contains

5.2810^1

or 52.8 cells/CFU we can plate this and expect 52 to 55 colonies or we can dilute this again by taking 1 ml in 9 ml for a ten fold dilution to have 5.28 cells

Which we will plate to expect 5-6colonies at which point we can approximately deduce that the original colony had 5.28 10^7 cells since we had 52.8 cells after diluting 100^3 or 1000000 times.