If you performed a Gram stain on a sample from pure culture of bacteria and observed a field of red and purple cocci, adjacent cells were not always the same color. What do you conclude?

If you performed a Gram stain on a sample from pure culture of bacteria and observed a field of red and purple cocci, adjacent cells were not always the same color. What do you conclude?

• A. The sample is contaminated with fungi
• B. A sample beyond 24-48 hours was used
• C. The staining procedure was done correctly but cells are spores
• D. The sample contains two species with identical Gram reaction

Answer: (B)

Gram staining shows uniform results when the culture is fresh, but aging can cause Gram-variable staining. As cells age, their cell walls can degrade or autolysis can occur, so some cocci will retain the crystal violet and appear purple while neighboring cells decolorize and take up the counterstain to look pink or red. This mixed staining pattern is a classic sign of an older culture, since the decolorization of some cells happens more readily as time passes. Therefore, observing both colors among cocci in a single field suggests the sample was used beyond the ideal window (beyond about 24–48 hours), rather than indicating contamination or the presence of spores or multiple species.

Gram staining shows uniform results when the culture is fresh, but aging can cause Gram-variable staining. As cells age, their cell walls can degrade or autolysis can occur, so some cocci will retain the crystal violet and appear purple while neighboring cells decolorize and take up the counterstain to look pink or red. This mixed staining pattern is a classic sign of an older culture, since the decolorization of some cells happens more readily as time passes. Therefore, observing both colors among cocci in a single field suggests the sample was used beyond the ideal window (beyond about 24–48 hours), rather than indicating contamination or the presence of spores or multiple species.