08 Jul

Capsular Staining

Capsular staining is a technique used to stain species of bacteria that are surrounded by an extracellular slime layer called a capsule or glycolayx.  This structure provides protection for bacteria such as streptococcus pneumoniae.  The capsule prevents phagocytic white blood cells from destroying the bacteria.  This allows the pathogen to enter the lungs and cause pneumonia.  The capsule also allows the bacteria to attach itself to solid surfaces.  Streptococcus mutans can attach to the surface of teeth which can cause dental plaque.  Most capsules are composed of polysaccharides but in some cases the capsule can consist of polypeptides with unique amino acids.  Staining of the bacterial capsule cannot be accomplished by ordinary staining procedures.  The capsule can be viewed by combining the simple stain with the negative stain.  The negative stain is performed first followed by slight heat fixing.  Crystal violet is then used to stain the bacterial cell.  The capsule appears as a halo around the cell.