Which statement best describes Gram-positive bacteria?

Which statement best describes Gram-positive bacteria?

• A. Have a cell wall with large amount of peptidoglycan and stain purple
• B. Have less peptidoglycan and will stain pink
• C. Lack peptidoglycan entirely
• D. Are always pathogenic

Answer: (A)

Gram staining tests how cell wall structure affects dye retention. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet–iodine complex during the decolorization step, so they appear purple under the microscope. This contrasts with Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane; after alcohol decolorization the dye is washed out and they take up the pink counterstain. Some bacteria, such as those lacking peptidoglycan entirely, do not fit neatly into the Gram-positive category. Pathogenicity is not determined by Gram type, since many Gram-positive organisms are harmless or beneficial, while some Gram-negative bacteria are nonpathogenic as well. Therefore, the statement describing a thick peptidoglycan wall and purple staining best describes Gram-positive bacteria.

Gram staining tests how cell wall structure affects dye retention. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet–iodine complex during the decolorization step, so they appear purple under the microscope. This contrasts with Gram-negative bacteria, which have a thinner peptidoglycan layer and an outer membrane; after alcohol decolorization the dye is washed out and they take up the pink counterstain. Some bacteria, such as those lacking peptidoglycan entirely, do not fit neatly into the Gram-positive category. Pathogenicity is not determined by Gram type, since many Gram-positive organisms are harmless or beneficial, while some Gram-negative bacteria are nonpathogenic as well. Therefore, the statement describing a thick peptidoglycan wall and purple staining best describes Gram-positive bacteria.