gramvärjäys

Gram staining is a differential staining technique used to classify bacteria into two broad groups: Gram-positive and Gram-negative, based on their cell wall composition. This method was developed by Hans Christian Gram in 1884. The process involves several steps, starting with applying a primary stain, typically crystal violet, to a heat-fixed smear of bacteria. This stain is absorbed by all bacterial cells. Next, a mordant, usually Gram's iodine, is added. This solution forms a complex with the crystal violet, trapping it within the cell wall.

Following the mordant, a decolorizing agent, commonly a mixture of alcohol and acetone, is applied. Gram-positive bacteria have a thicker peptidoglycan layer in their cell walls, which retains the crystal violet-iodine complex and appears purple or blue. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer lipid membrane, which is disrupted by the decolorizing agent, causing the crystal violet-iodine complex to be washed out.

Finally, a counterstain, such as safranin, is applied. This stains the decolorized Gram-negative bacteria pink or red, while the Gram-positive bacteria, already stained purple, remain that color. The appearance of the bacteria under a microscope after Gram staining reveals their Gram-positive or Gram-negative status. This distinction is crucial in microbiology and clinical diagnostics, as it often correlates with antibiotic susceptibility and can guide initial treatment decisions. The Gram stain is a fundamental tool for identifying and categorizing bacteria.