Stains for Light Microscopy

The Gram stain was developed by Christian Gram in 1884 and modified by Hucker in 1921. The Gram stain separates bacteria into two groups: Gram-positive microorganisms that retain the primary dye and Gram-negative microorganisms that take the color of the counterstain. Details of the three-step Gram stain by Meszaros and Strenkoski are available. Most clinical laboratories are currently using the four-step Gram stain. A modified Brown and Brenn Gram stain can be used to detect Gram-negative and Gram-positive bacteria in tissue. This is the major reason why the bacteria on the edge of a Gram smear are more likely to give true results, and those in the center are often unreliable for reading a Gram stain. The bacteria may have an atypical morphology and Gram staining can give atypical results. The stain maybe useful in interpreting thick purulent specimens that failed to give clear results with the Gram stain.