Question

Imagine a student measures the concentration of a bacterial culture using the "direct microscopic method" (using a hemocytometer) and by "using turbidity to count cells in liquid culture." Based on their calculations, there were 10,000 total cells/mL and the viability of the culture was 90%. Which method allowed the student to determine viability and why/how?

Answer 1

Viability of bacterial cells can't be determined from turbidity. Because dead cells are also present in culture along with live cells, and these two are almost similar morphologically. Thus viable cells are actually less in number than total cells present.

Direct microscopic count using Hemocytometer is one of the methods to know viable cell number in a culture. There are some dye like Trypan blue etc. that can differentiate between live and dead cells. For example, this dye can't colour live cell with intact cell membrane, but stains dead cells. This difference can easily be detected under microscope.

The culture is first stained with such dye and then observed under microscope. Both total number of cells and viable cells can be determined easily.

In this case, if we get 10000 total cells/mL of culture and in the same culture if viable cells are 9000/mL in number, then we can say 90% of the total cells are viable.