In the typical Gram stain a Gram-negative bacteria will stain red. The bacteria may be unstained or be difficult to differentiate from the background if certain conditions are present.
Possible causes:
(1) section was left too long in the dehydrant after the red stain (fuchsin)
(2) inadequate exposure to the dehydrant
(3) counterstain was not used to provide contrast (if used)
(4) problem with the red stain
(5) incomplete removal of the crystal violet-iodine complex
Possible solutions:
(1) control the time that the section spends in the dehydrant
(2) increase the time in the organic solvent to completely remove the crystal violet-iodine complex
(3) prepare fresh red dye
(4) quickly decolorize the counterstain
Always use a control slide when staining to make sure that both Gram-negative and Gram-positive bacteria can be discerned.
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