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Microbiology Questions- Direct and Viable Counts

The depth of the hemocytometer sample box is 0.1mm for all questions.

Please show all calculations.

1. Why might the direct and variable counts of a suspension disagree?
2. You dilute an original sample 1:30. You then count the number of yeast in the 1:30 dilution. You count an average of 7 cells in the 1/25 mm^2 sized boxes. What is the density, in cells/ml, of the original sample?
3. You count the number of bacteria in 5 of the 1/400 mm^2 boxes of the central grid on the hemocytometer. Your results are: 12 cells in box #1, 17 cells in box #2, 17 cells in box #3, 14 cells in box #4 and 16 cells in box #5. You are counting a 1:10 dilution of the original sample. What is the density of the original culture (in cells/ml)?

Solution Preview

1. This is my assumption: I believe it is "viable" counts instead of variable counts. The response below is based on that assumption.

The direct count of a suspension involves counting the number of visible cells in an area under a microscope. The direct count does not distinguish between live or dead cells in a suspension. In contrast, the viable count of a suspension (such as bacteria) generally involves plating the cell suspension on a solid media to allow formation of colonies. Each colony arises ...

Solution Summary

Direct and viable counts for microbiology questions are examined.