What Is The Gram stain?
By Tim Sandle
The primary staining technique used to differentiate bacteria is the Gram stain. The Gram stain method employed includes the four-step technique: Crystal violet (primary stain); iodine (mordant); alcohol (decolorizer); safranin (counter stain) or the three-step method in which the decolorization and counter-staining step are combined. Done correctly, Gram-positive organisms retain the crystal violet stain and appear blue; Gram negative organisms lose the crystal violet stain and contain only the counter-stain safranin and thus appear red. Common pitfalls in this method are that heat fixation may cause Gram-positive cells to stain Gram-negative and older cultures may give Gram-variable reaction; using too much decolorizer could result in a false Gram-negative result and not using enough decolorizer may yield a false Gram-positive result.
In addition to the difference based on cell wall, microscopic examination of the stains allows the cellular shape to be determined. Bacteria commonly fall into the categories of coccus (spherical), rod, vibrio (curved), spirilla (spiral) and plemomorphic (variable).
Posted by Tim Sandle
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