Microbiology Lab Practical Practice Exam

Non-acid-fast cells take up the counterstain and appear blue or green after the decolorization step, because they don’t retain the red dye used as the primary stain. Their cell walls lack the waxy mycolic acids that make acid-fast organisms resistant to decolorization, so they are cleared of the primary stain and show the counterstain color. Pink or red indicates acid-fast organisms, which hold onto the red dye even after acid-alcohol treatment. Purple would not be the typical color for this differential staining and is more associated with Gram staining.