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Chapter 3.2 : Staining Procedures
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The Gram stain has many uses: principally, it classifies bacteria on the basis of their cell wall structure and allows observation of their size and cellular morphology as well. It can also be utilized to assess the quality of clinical specimens and as a critical test for rapid and presumptive diagnosis of infective agents directly from specimens ( 1 , 2 , 3 ). The stain was originally developed by Christian Gram in 1884 ( 4 ). The modification currently used for general bacteriology was developed by Hucker in 1921, providing improved consistency of decolorization and better differentiation of organisms ( 5 ). Kopeloff’s modification, which employs a basic fuchsin (or carbol fuchsin) counterstain ( 6 ), has particular utility for staining anaerobes and weakly staining Gram-negative organisms (e.g., Legionella, Campylobacter, and Brucella) ( 7 , 8 ). Consequently, many laboratories use these counterstains routinely, especially for direct smears of clinical material.
Touch preparation of smear.
Typical Gram stain morphologies of Gram-positive and Gram-variable genera.
Typical Gram stain morphologies of Gram-negative genera.
Gram stain modifications, recommended reagents, timing, and uses
Gram-positive organisms found in direct smears from clinical sources
Gram-negative organisms seen in direct smears from clinical sources
Common descriptions of bacterial Gram staining characteristics
Reporting Gram stain results a
Standardized scoring method for evaluation of Gram stains for BV
Outline for reporting wet mount results from different specimens