What are the common cell labeling methods?

Four of the most common cell labeling methods include gram staining, fluorescent dyes, immunolabeling, and fluorescent fusion proteins. 

• Gram staining is a simple cell labeling technique that’s used to differentiate between two large groups of bacteria, Gram positive and Gram negative bacteria, based on the composition of their cell wall. Gram positive bacteria, which have a thick layer of peptidoglycan in their cell walls, turn violet when treated with the Gram stain. This is because the thicker peptidoglycan layer retains the crystal violet stain during the decolorization process. Gram negative bacteria, which have a thinner layer of peptidoglycan in their cell walls, stain red when treated with the Gram stain. This is because the thinner peptidoglycan layer loses the crystal violet stain and instead takes on the safranin stain in the final staining step. 
• Fluorescent dyes are biological molecules that are composed of at least one fluorophore that gets excited by light energy, first absorbing the light and then emitting it to release the extra energy it has absorbed.  Fluorescent dyes may occur naturally or they can be made synthetically. Fluorescent dyes offer high sensitivity and specificity and can be used for both live and fixed cells. They are also photostable and long-lasting and can be multiplexed for simultaneous labeling of multiple targets. 
• Immunolabeling is a cell labeling method in which a biological target is labeled with an antibody. It is categorized as two types – direct and indirect immunolabeling. 
• Direct immunolabeling uses only one antibody that is able to recognize the target molecule and emit light on excitation. This immunolabeling method is more sensitive and shorter protocol than indirect staining. However, background staining can be a problem.
• Indirect immunolabeling uses two antibodies, one without and one with a fluorophore. The antibody without a fluorophore binds to the target molecule and the antibody with a fluorophore binds specifically to the primary antibody and is visible when excited. This immunolabeling method has higher specificity and more flexibility than direct staining. However it has a longer protocol, and is more expensive. 
• Fluorescent fusion proteins are made up of a fluorescent protein that’s bound to another protein of interest. They change color through the course of a mechanism, enabling the study of proteins in real-time in living cells and organisms. Fluorescent fusion proteins are noninvasive and can be used for live imaging. They offer the advantages of high specificity and high sensitivity.