Question

Why does nigrosin not stain bacterial cells/structures?

A) Nigrosin repels bacteria.
B) Bacteria have a positive charge, repelling nigrosin.
C) Nigrosin only stains the cell wall.
D) Nigrosin is too diluted to stain bacteria.

Answer 1

Nigrosin does not stain bacterial cells or structures because it is a negatively charged dye that repels the negatively charged surfaces of bacterial cells. Negative staining with nigrosin is used to visualize capsules, as the stain colors the background, leaving the capsules as halos around the cells.

Step-by-step explanation:

Nigrosin does not stain bacterial cells or structures because it is a negatively charged dye, while bacterial cells have a negatively charged surface. The negative charges of both the dye and the bacterial cell repel each other, preventing the dye from binding to the cell. As a result, the cells remain unstained.

Negative staining with India ink or nigrosin is often used to stain the background of the slide, leaving bacterial cells and structures clear. This technique is commonly used to visualize capsules, which are protective outer structures of certain bacteria and yeasts. The negative stain colors the background, making the capsules appear as halos around the cells.

An example of negative staining is when nigrosin is used to stain the background around yeast cells of Cryptococcus neoformans. The dye does not penetrate the encapsulated cells, causing the capsules to appear as light-blue halos.