India ink, Congo red and Eosin may be used as alternative dyes in place of Nigrosin for negative staining. India ink works well, but its tiny particles cause Brownian movement, which makes identifying the bacteria a bit difficult.
Negative stains do not stain bacterial cells, which are negatively charged, but the background around the cells. Under the microscope, the cells appear as colorless outlines against a dark background. This staining method is used for studying specimens that must not be fixated with alcohol or heat, so that the viewing of a living cell is possible.
Some bacteria have extra-cellular structures called capsules, which are easily destroyed by washing and heating. To identify if the sample bacteria have capsules, a mild technique using negative stain is employed.