GramFärbungen

GramFärbungen, also known as Gram staining, is a common differential staining technique used in microscopy to classify bacteria based on their cell wall composition. Developed by Danish bacteriologist Hans Christian Gram in 1884, this method is crucial for initial identification and guiding treatment decisions for bacterial infections.

The process involves several steps. First, a heat-fixed smear of bacteria on a microscope slide is treated with a primary stain, typically crystal violet. This dye stains all bacterial cells purple. Next, a mordant, usually iodine solution, is applied. The iodine forms a complex with the crystal violet, making it less likely to be washed out. The slide is then decolorized with a solvent, most commonly an alcohol or acetone-alcohol mixture. This is where the differentiation occurs. Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet-iodine complex and appears purple. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; the decolorizer washes out the crystal violet-iodine complex, rendering them colorless. Finally, a counterstain, such as safranin or carbolfuchsin, is applied. This stains the decolorized Gram-negative bacteria pink or red, making them visible under the microscope. Gram-positive bacteria remain purple because the primary stain is not removed. Therefore, GramFärbungen reliably distinguishes between these two major bacterial groups, providing valuable information for microbiology and clinical diagnostics.