What does it mean to have a countable plate? In roughly what dilution would you

Question

What does it mean to have a countable plate? In roughly what dilution would you expect to find a countable plate?

You are given a sample of unknown Bacilus subtilus and need to find out the cell concentration of the original sample. Describe your experimental design. Draw out a dilution map that illustrates your experimental design. Be sure to label the amount of autoclaved water going in to each vial, volume of bacteria sample transferred, the plating volume and determine the resulting concentration that it will produce after being plated.

What is the original cell density of a sample that produced 117 colonies on a plate that was inoculated with a 0.1mL sample from a tube containing a 10-5 dilution?

What is the original cell density of a sample that produced 32 colonies on a 10-7dilution plate?

If you only have three agar plates left that are suitable to grow V. fischeri and your original cell sample was 2.6x105, which three dilution vials would you inoculate in order to guarantee yourself a countable plate? Assume the inoculation volume used on each plate was 0.1mL and that each vial had a dilution factor of 10.

Explanation / Answer

1). Countable plate means, the number of colonies in a microbial growth plate must be in range of 25 to 250 or 30 to 300 colonies. So that there will be no overlap of colonies, which allows easy counting.

The final dilution of plate must be in range of 25 to 250 or 30 to 300 colonies as said above, the number of dilutions needed to achieve this plate count depends on the concentration of stock solution.

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