Gram-negative bacteria appear what color after Gram staining?

A. Their outer membrane traps the stain: Gram-positive bacteria do not have an outer membrane; this structure is characteristic of Gram-negative bacteria. Therefore, the outer membrane cannot be responsible for retaining crystal violet in Gram-positive cells.

B. Their thick peptidoglycan layer holds the crystal violet-iodine complex: Gram-positive bacteria have a thick, multilayered peptidoglycan cell wall that forms a dense network. During Gram staining, the crystal violet combines with iodine to form an insoluble complex that gets trapped within this thick peptidoglycan matrix. Even after alcohol or acetone decolorization, the complex remains, causing the cells to appear purple under a microscope.

C. They absorb the safranin more strongly: Safranin is a counterstain used to color Gram-negative bacteria red after decolorization. Gram-positive bacteria do not absorb safranin strongly because the retained crystal violet-iodine complex masks it. Therefore, safranin absorption is not responsible for the purple color of Gram-positive cells.

D. They do not undergo decolorization: While Gram-positive bacteria resist decolorization, this is not because they fail to undergo the process; it is due to the structural ability of their thick peptidoglycan to trap the crystal violet-iodine complex. The resistance is a result of the cell wall structure, not a lack of exposure to the decolorizing agent.

A. Safranin: Safranin is a basic dye that is primarily used as a counterstain in differential staining techniques such as the Gram stain and endospore staining. It provides contrast to differentiate between Gram-positive and Gram-negative bacteria or to highlight structures like spores, but it is not typically used alone for simple staining.

B. Methylene blue: Methylene blue is a cationic (basic) dye that binds electrostatically to negatively charged components of bacterial cells, such as nucleic acids and cell walls. Its use in simple staining allows clear visualization of cell morphology, size, and arrangement under a light microscope. This makes it a reliable and widely accepted stain in microbiology for rapid identification of bacterial cellular features.

C. Malachite green: Malachite green is a primary stain used in endospore staining procedures because it can penetrate the tough keratin coat of bacterial spores, usually with the aid of heat. It is not suitable for simple staining as it does not readily stain vegetative cells and requires additional steps, making it more specialized.

D. Carbol fuchsin: Carbol fuchsin is a phenolic dye used in acid-fast staining to identify mycobacteria, which have waxy, lipid-rich cell walls resistant to conventional stains. Its medical significance lies in tuberculosis and leprosy diagnostics, but it is not used for simple staining because its application requires heat and decolorization with acid-alcohol.