Bacteria do not show much structural detail under light microscope due to lack of contrast. So bacteria must be stained to produce color contrast. Bacteria may be stained in the living state or killed state. Staining in which bacteria are in the living state is called as vital staining. If the bacteria are killed during staining, the process is called as supravital staining.
1. Simple staining:
Stains used in this technique are dyes such as methylene blue or basic fusion. These dyes provide color contrast, but impart the same color to all bacteria.
2 Negative staining:
In this technique, bacteria are mixed with dyes like Indian ink or ligroin. These dyes provide a uniformly collared background. The unstained bacteria can be seen against the collared background. This technique is useful for demonstrating bacterial capsules which do not taw simple stains.
3. Impregnation methods.
Cells and structures which are too thin cannot be seen under ordinary microscope. These structures can be made visible if they are thickened by impregnation of silver on the surface. Spirochetes and bacterial flagella can be demonstrated by impregnation methods.
4. Differential staining:
These stains impart different colors to different bacteria or bacterial structures. The two differential stains commonly used are: 1 Gram stain 2. Acid fast stain.’