Question

I did a direct count to figure out my yeast solution concentration and got 8.37 x 10^6 for my direct count using a hemocytometer. I then diluted this yeast sample, plated 0.23ml of the 1/100000 diluted sample of my agar plate and after a week of allowing colonies to grow, I calculated a viable count of 7.60 x 10^6. Do the direct and viable counts of the suspension agree? Why or why not?

Answer 1

The slight disagreement of the cell count by 1.16 *10^6 cells is due to the fact that in the hemocytometer the dead cells cannot be differentiated from the live cells unless stained with appropriate dyes whereas in plate count only live cells give rise to colonies. Therefore, direct count gives a greater number as it gives the count of live cells+dead cells whereas viable count is that of live cells only. Another reason for lesser number of cells in viable count may be due to manual errors while performing dilution and plating.