Cell Staining

Medical science depends on the staining of cells in tissues to make accurate diagnoses of a wide range of diseases from cholera to sexually transmitted diseases, to parasitic diseases and skin infections. Staining techniques performed routinely in microbiological laboratories include gram's stain, acid-fast stains, acridine orange, calcofluor white, toluidine blue, methylene blue, silver stains, and fluorescent stains. Stains are classified broadly as basic, acidic, or neutral stains. The chemical nature of the cells under examination determines which stain is selected for use.

Cell staining is important in the diagnosis of microorganisms because bacteria can be identified by the color differentiation of stains (dyes). Microscopic examination of stained cell samples allows examination of the size, shape, and arrangement of organelles, as well as external appendages such as the whip-like flagella, which are the cell's organs of motion. When sample cells are stained to show their chemical composition it is called differential staining.

Histochemistry is the specialty that studies the staining properties of cells. Histochemistry is used in other specialties such as histology (the study of tissues), biochemistry (the study of the chemical makeup of cells), cytology (the study of cells), and microbiology (the study of organisms that are too small to be seen without a microscope).

In 1880, Hans Christian Gram of Denmark noted the differences in the way bacteria react to stains. Those bacteria that retained a deep purple stain, even after they were washed, were termed "stain positive." Those that lost the stain and responded again to another stain, were termed "stain negative." Today, bacteria are classified as "gram-positive" or "gram-negative" to distinguish the two major groups of bacteria. This staining test highlights differences in the structure of the cell wall of the two types of bacteria.

Penicillin G is used to treat gram-positive infections, but it is ineffective against gram-negative bacteria. Other antibiotics are only effective against gram-negative bacteria. Chloromycetin, which was discovered in 1947, was the first antibiotic to be effective against both gram-positive and gram-negative bacteria.