Characterisation by staining: Capsular staining
Sometimes it is convenient to determine overall bacterial morphology without the use of harsh staining or heat-fixing techniques that change the shape of cells. This might be the case when the bacterium does not stain well (e.g., some of the spirochetes) or when it is desirable to confirm observations made on the shape and size of bacteria observed in either a wet-mount or hanging drop slide. Negative staining is also good for viewing capsules.
Negative, indirect, or background staining is achieved by mixing bacteria with an acidic stain such as nigrosin, India ink, or eosin, and then spreading out the mixture on a slide to form a film. The above stains will not penetrate and stain the bacterial cells due to repulsion between the negative charge of the stains and the negatively charged bacterial wall. Instead, these stains either produce a deposit around the bacteria or produce a dark background so that the bacteria appear as unstained cells with a clear area around them.