1.) Why is purple dye removed from gram negative cells, but not from gram positi

Question

1.) Why is purple dye removed from gram negative cells, but not from gram positive cells?
2.) Explain molecularly/structurally why a pure culture that has been growing for a very long time might have some inaccurate results (e.g. it stains pink when the species is actually gram positive)

These questions are from a microbiology course. 1.) Why is purple dye removed from gram negative cells, but not from gram positive cells?
2.) Explain molecularly/structurally why a pure culture that has been growing for a very long time might have some inaccurate results (e.g. it stains pink when the species is actually gram positive)

These questions are from a microbiology course.
2.) Explain molecularly/structurally why a pure culture that has been growing for a very long time might have some inaccurate results (e.g. it stains pink when the species is actually gram positive)

These questions are from a microbiology course.

Explanation / Answer

1. The purple stain or crystal violet imparts that color to gram positive cells which contain high amounts of peptidoglycan. The gram negative cells do not contain peptidoglycan in high amounts. They contain lipid layer which is somewhat porous. Hence, when the cells are washed, the gram negative cells lose the purple dye and take up the secondary stain saffranine and appear pink.

2. Pure culture stored for a long time loses its intactness of cellwall and plasma membrane. Hence, after the wash they lose the purple dye and though the culture is a gram positive it appears pink sometimes.

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