Question

[CH-15-09]

Why do living cells appear colorless and dead cells stain blue during direct microscopic count in samples mixed with methylene blue?

Answer 1

Reason that the living cells appears colorless while the dead cells stain blue.

Methylene blue in reduced state is colorless while in its oxidised state it is blue. A viable living cell will take up the stain and these reduce methylene blue stain to a colorless product. This is because the living cells possess an active dehydrogenase system that reduces the dye methylene blue and thus cells appear colorless against the blue background. On the other hand dead cells will be unable to reduce the stain and thus appears blue or black in colour.