Typically gram-______ cells.

• A. Positive
• B. Negative
• C. Neutral
• D. Gram-variable

Answer: (B)

Gram staining differentiates bacteria by their cell wall structure, which determines how they hold or lose the crystal violet stain. Gram-negative cells have a thin peptidoglycan layer and an outer membrane; during the staining process, the alcohol decolorizer disrupts this outer layer and washes out the violet, so these cells take up the contrasting counterstain and appear pink or red. Gram-positive cells, with their thick peptidoglycan layer, trap the violet-iodine complex and stay purple. Because many organisms encountered in practice are Gram-negative, the typical completion of the sentence is negative. Note that some bacteria can appear Gram-variable when the culture is old or conditions affect the cell wall, but that is the exception rather than the rule.