Negative staining

Negative staining of bacteria.

Negative staining provides the simplest and the quickest method of gaining information about the cell shape, cell breakage and refractive inclusions in cells such as sulphur and β- hydroxyl butyrate granules and about endospores. In negative staining a simple stain is used that does not stain the bacteria but stains the background. Nigrosin or India ink , and acidic stain is used. The acidic stain carries a negative charge, is repelled by the bacteria which too carry a negative charge on their surface, therefore, bacterial cells appear transparent and unstained upon examination. A stain that stains the background and does not stains the bacteria is called negative stain. Negative staining is advantageous for two reasons: Cells appear less shrivelled or distorted because no heat fixing is done.

2. Capsulated bacteria that are difficult to stain can be observed by this technique.

Cultures used should be 24 hr. old. 

• Always place a small drop of stain close to one end of a clean slide. 


• ·Do not spread the drop during mixing it with the culture. 


• The thickness of the film should be uniform. 


• Never heat the slide too much. 


• Negatively stained preparations should not be used for cell length and width measurements.