Bacteria that stain pink with a Gram stain are referred to as?

• A. Endospore-forming
• B. Gram negative
• C. Bacillus
• D. Acid-fast

Answer: (B)

Pink staining in a Gram stain indicates Gram-negative bacteria. This result comes from their cell wall structure: a thin peptidoglycan layer plus an outer membrane. During staining, crystal violet-iodine complexes are washed out by the alcohol, and the thin peptidoglycan can’t hold the dye, so the counterstain makes the cells appear pink. In contrast, Gram-positive bacteria have a thick peptidoglycan layer that retains the purple dye, so they look purple. The other terms don’t describe this color outcome: endospore-forming refers to the ability to produce spores, not the Gram reaction; Bacillus is a genus typically Gram-positive; acid-fast organisms resist Gram staining and require a different staining method.