A method for differential staining of bacteria; smears are fixed by flaming, stained in a solution of crystal violet, treated with iodine solution, rinsed, decolorized, and then counterstained with safranin.
In 1884, Hans Christian Gram, a Danish doctor working in Berlin, accidentally stumbled on a method which still forms the basis for the identification of bacteria. While examining lung tissue from patients who had died of pneumonia, he discovered that certain stains were preferentially taken up and retained by bacterial cells. Over the course of the next few years, Gram developed a staining procedure which divided almost all bacteria into two large groups - the Gram stain. Individual bacterial cells are hard to see, partly because they are small, but also because they are almost transparent. In addition to magnification under a microscope, optical tricks must also be used to be able to see them: · Phase contrast microscopy · Staining Either of these methods can make bacterial cells visible under the microscope. Other staining methods are described elsewhere in these documents, e.g. the Ziehl-Neelsen acid-fast staining procedure, but the Gram stain procedure is as follows: