Question

2 Unit 1 The Fundamentals ADDITIONAL QUESTIONS 1 a. What is the advantage of a spread plate count over a simple hemocytometer count? b. Given your answer to (al, why would you do a hemocytometer count that is what is the advantage of this method over spread plating? 2. Why are counts expressed in CFU per milliliter instead of cells per milliliter? 3. If you want a final cell density in CFU per 10 ml, why do you apply only 0.1 ml to the spread plate?

Answer 1

1.a )In this case, the spread plate allows you to count the cells directly on a culture plate. Of course, with the difference that you are counting CFU in the spread plate, it is an easy technique that does not require an hemocytometer and a microscope. In the hemocytometer, you need a low diluted sample in order to count individual cells under the microscope. This is hard if we are talking about bacterial cells since they are mostly grouped in colonies.

1.b) The hemocytometer allows you to determine viability of cells. This is most common in eukaryotic cell cultures. However, with the hemocytometer we can look the morphology in more detail and predict if your culture is viable or not.

2) Before answering, you need to understand what CFU means. CFU means Colony Forming Unit which means that you are considering only one cell of each colony. We are not able to count each cells since one colony has a huge amount of individual cells.

3) In this case, you are making a 10 fold dilution, 0.1 ml is one part of 1.0 mL. So, you will have a dilution factor 1/10. This will allow you to dilute your sample the times you want in order to be able to count each colony.