Why does nigrosine stain background and not the cell wall in the procedure?

Nigrosine is an acidic dye that carries a negative charge, which repels from the negatively charged cell wall of bacteria. Instead, it stains the background by providing a contrast to the cells, as it does not penetrate or adhere to the cells themselves. This results in a clear visualization of the cells against a dark background, allowing for easier observation of cell morphology.