What is the principle of indirect staining?

Indirect staining is a staining technique that stains the background of cell smears rather than staining the organism. This staining technique uses a negative stain, which is unable to penetrate the cell because of the repulsion between the negative charges of the bacterial surface and the stain. Instead, it stains the background around the cell, yielding a clear cell with a dark halo around it. Indirect staining is a widely used technique for viewing the size and shape of bacterial cells.