Here's why:
* Methylene blue is a redox indicator: It can exist in two states: oxidized (blue) and reduced (colorless). When methylene blue comes into contact with a reducing environment (like a cell), it gets reduced and turns colorless.
* Live yeast cells have active reductase enzymes: These enzymes can reduce methylene blue, making it colorless. This is why a solution of methylene blue will appear clear when added to a culture of living yeast.
* Dead yeast cells lack active reductase enzymes: Since the enzymes are no longer functional, they cannot reduce the methylene blue, so it remains blue.
Therefore, the presence of blue color indicates the absence of active reductase enzymes, which is a sign of cell death.
However, it's important to note that:
* The dye itself doesn't kill the yeast: It's just a visual indicator of cell viability.
* Methylene blue can be toxic in high concentrations: It can interfere with cellular processes, leading to cell death.
In summary, methylene blue can be used to distinguish between live and dead yeast cells because it's reduced by active reductase enzymes in live cells, making it colorless. Dead cells lack these enzymes, so the dye remains blue.
