Nigrosin is an acid stain. The negative charge of the acid stain and the negative charge of the cell will repel each other. In a Negative stain with Nigrosin, the cells appear clear against a dark (stained) background. When performed well this can be a beautiful stain and can give you a feeling of accomplishment and even wonder when you first see the cells pop against the background. The staining procedure is very different from the other stains done in the course. First of all, the normal smear prep is not performed, and the slide is NOT heat-fixed. The specimen is mixed with a small drop of the Nigrosin, spread across the slide via a second slide, and then air-dried. This is a very mild treatment of the cells, which serves to preserve their shape and size. Thus, the Negative stain is very good for observing cell morphology, arrangement, and size. This can be very helpful when it is difficult to determine these characteristics with other stains. However, because the slide is not heat-fixed, the bacterial can remain viable and it is important to be careful about handling and disposal. It is very important that you do not touch the slide to the objective of the microscope; therefore, you must be very careful with focusing while using oil immersion since the working distance is so small.
Contributors and Attributions
Kelly C. Burke (College of the Canyons)