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at the end of the staining scheme; these are called gram-positive bacteria; and (ii) bacteria which acquire pink to red color and are designated
gram-negative bacteria.
The Gram stain is by far the most widely applied staining technique in all microbiology disciplines’ laboratories. It is one of the most important
identification criteria in any identification scheme for all types of bacterial isolates. Knowing the Gram reaction of a microorganism, in addition to
morphology characterization, rules out a huge number of other bacterial candidates thus reducing unnecessary labor and time that could have
been otherwise wasted in identification of unrelated bacterial types.
The technique involves the use of :
A primary stain, most often crystal violet (CV), as the blue-violet dye.
A mordant (originally meant caustic), a solution of iodine that forms a complex with CV in stained bacterial cells.
A decolorizing solution, a solvent (alcohol or alcohol-acetone mixture) that removes color of CV from some bacterial types but not others.
A counterstain, most often safranin, which confers red color on decolorized cells, thus contrasting the purple color of cells that retain CV after
decolorization.
This presentation is a Flash-animated movie that demonstrates in a simple graphical illustration the step-by-step staining procedure based on Gram’
s method. A time-line screen insert also shows synchronously the staining events that take place at higher magnification inside the bacterial smear.
Further credits:
Authors
Hussein Shoeb
Department of Microbiology & Immunology
Faculty of Pharmacy, Cairo University
Cairo
Egypt
Email: shoeb@bostany.com
© American Society for Microbiology, Washington DC