The development of microscopy and cell-staining techniques made possible impressive advances in our understanding of microorganisms. That fact should hardly be surprising. The microscope allows one to see structures too small to be visible with the naked eye; individual cells, for example. But those small structures are usually colorless and transparent, making it difficult to distinguish specific features contained within them. Thus, it became important to find ways of bringing out these features so that they could be studied in more detail. The father of microscopy, Antoni van Leeuwenhoek, found that dyeing muscle fibers with saffron allowed him to see the detailed structure of the fibers more easily.
From van Leeuwenhoek's time to the mid-nineteenth century, scientists searched for natural products to use in dyeing cells. They were only moderately successful until the discovery of the first synthetic dye by William Perkin in 1856. Then the new technology developed by Perkin unleashed a flood of colored compounds to the scientific community for potential use as stains.
One of the first scientists to explore these new dyes was the German anatomist Walther Flemming. Flemming used a number of different dyes to stain cells and observed structures that seemed to absorb these dyes strongly. Among these structures were bodies that he eventually called chromatin, after the Greek word for "color."
During the late 1800s, the art of cell staining became much more sophisticated. The German biologist Carl Weigert (1845-1904) observed, for example, that different kinds of bacteria are stained by different dyes. That discovery turned out to be a powerful influence on Weigert's cousin, the famous bacteriologist Paul Ehrlich. Ehrlich wrote his final college thesis on the techniques of cell staining. During his residency at the Charité Hospital in Berlin, Germany, Ehrlich discovered techniques for identifying blood disorders based on the way that cells absorb dyes. He also developed methods for staining white blood cells and mast cells.
Ehrlich's work on stains eventually led him in a quite different direction. He became convinced that staining might not be a neutral process in which cells simply absorb dye. There might, he thought, be some actual physical or chemical interaction between cell and dye that could result in the death of the cell. Thus, he began the search for a dye that would kill harmful bacteria in the process of staining.
Over a period of more than a decade, Ehrlich worked toward this objective. He finally found a dye called trypan red that kills trypanosomes, the protozoa that cause sleeping sickness. In 1907, he found an even more important bactericide called Salvarsan. This arsenic-containing compound proved to be a powerful agent in the treatment of syphilis.
Perhaps the most famous stain discovery was made in 1884 by the Danish bacteriologist Hans Christian Joachim Gram. Gram modified a traditional staining technique by adding iodine and then an alcohol wash to stained cells. He found that some cells retained the original dyes in this procedure, while others lost their color. The former were designated gram positive cells and the latter, gram negative cells. The procedure has since become a standard method for classifying bacteria.
Another approach to staining was suggested by the Italian histologist Camillo Golgi in the 1870s. Instead of using organic dyes, Golgi experimented with the use of silver salts as stains. He found that cell structures not visible with organic dyes were now easily seen. One of the first structures he discovered is now known as the Golgi body.
Yet another technique is immunoperoxidase staining, in which cells are washed with antibodies to various cell parts or cell chemicals. The antibodies are tagged with a dye that can be seen under a microscope, allowing observers to see certain components of the cell.
Today dozens of stains, both organic and inorganic, have been developed for specific uses. The names of many--Borrel's methylene blue, Ehrlich's triacid stain, Renault's eosin, Lugol's iodine, and Van Gieson's stain, for example-continue to memorialize their inventors.
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