Which statement about specimen staining is true?

• A. Anaerobic bacilli and other thick-walled, gram-positive bacteria stain weakly and appear pale red or pink.
• B. Gram-negative enteric bacteria have weak avidity to the safranin stain and appear pale pink.
• C. Pseudomonads have strong avidity to and uptake the maximum amount of safranin.
• D. Gram-positive bacteria appear dark violet, purple, or blue in color.

Answer: (D)

Understanding Gram staining outcomes based on cell wall structure helps explain why this statement is true. In Gram staining, Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet–iodine complex during the alcohol wash, so they appear dark violet, purple, or blue under the microscope. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; the alcohol step disrupts the outer membrane and allows the crystal violet to wash out, so these organisms take up the counterstain and appear pink or red. Pseudomonads are Gram-negative, so they would appear pink from the safranin counterstain, not the dark purple color of Gram-positive organisms. The other statements mischaracterize staining behavior: thick-walled Gram-positive bacteria do not stain pale red or pink, Gram-negative enteric bacteria do not have weak avidity to safranin (they just appear pink after the counterstain), and the idea of “maximum uptake” of safranin by pseudomonads isn’t how staining results are described.