Bacterial Staining

Bacterial Staining

Visualization of microorganisms in the living state is quite difficult, not only because they are minute, but also because they are transparent and practically colourless when suspended in an aqueous medium.  To study their properties and to divide microorganisms into specific groups for diagnostic purposes stains and staining procedures in conjunction with light microscopy have become a major tool in microbiology.

Chemically, a stain or dye may be defined as an organic compound containing a benzene ring plus a chromophore and auxochrome group.

The ability of the stain to bind to macromolecular cellular components such as proteins or nucleic acids depend on the electrical charge found on the chromogen portions as well as on the cellular component to be stained. 

Acidic dyes are anionic, which means that on ionization of the stain, the chromogen portion exhibits a negative charge and therefore has a strong affinity for the positive constituent of the cell.  Proteins, positively charged cellular components, will readily bind to and accept the color of the negatively charged anionic chromogen of an acidic stain.  Example for acidic dyes are picric acid and nigrosine.

Basic dyes are cationic because on ionization the chromogen portion exhibits a positive charge and therefore has a strong affinity for the negative constituents of the cell.  Nucleic acids, negatively charged cellular components, will readily bind to and accept the color of the positively charged cationic chromogen of a basic stain.  Examples for basic stain is Methylene blue.

Basic dyes are more commonly used in bacterial staining.  The presence of negative charge on the bacterial surface acts to repel most acidic dyes and thus prevent their penetration into the cell.  Numerous staining techniques are available for visualization, differentiation and separation of bacteria in terms of morphological characteristics and cellular structures.