The cells of most bacterial cells carry a negative charge that repels nigrosin. This microbiological stain is acidic and readily gives up hydrogen ions to become negatively charged. Nigrosin colors the neutral glass of the microscope slides but does not stain the cells, causing them to show up as a clear spot against a dark background, according to Microbugs.
Nigrosin offers the advantage of staining bacteria without the use of heat. Other stains commonly used in the microbiology lab, including methylene blue and carbol fuchsin, are base stains, so they are attracted to the bacteria and give it color while leaving the slide clear. However, the use of these stains requires heat. Nigrosin is especially useful with organisms unable to withstand such heat.
Nigrosin staining allows a microbiologist to determine the size and arrangement of the cells being viewed. It offers the opportunity to count the number of cells in the sample that otherwise would not show. It is a fast and simple process that offers quality results while preserving specimen integrity.
According to Reference.com, Nigrosin is also known as India ink. Artists use this ink in calligraphy, drawing and as a black tattoo ink. Manufacturers use it in some brands of black shoe polish.