Paul Ehrlich: from aniline dyes to the magic bullet

George Dunea
Chicago, Illinois, United States


Fig. 1, Phenylamine 

To understand Paul Ehrlich, the man who developed the first effective cure for syphilis, we must dial back to 1826. In that year, a German scientist called Otto Unverdoren isolated from indigo a volatile organic substance that smelled like rotten fish. Other scientists followed him and claimed to have isolated similar substances from coal tar some giving a beautiful blue color when treated with the appropriate chemical. In 1843, a German professor, August von Hoffman, showed that these various substances were all one and the same and named the compound aniline. It consisted simply of a six-ring carbon (phenyl) group attached to an amino group, (hence also called phenylamine, figure1) and when treated with the appropriate chemical it would yield many interesting substances. So it came about that in 1856 an eighteen-year-old

Fig. 2, Gown dyed with mauvine

British chemistry student, William Henry Perkins, was assigned during his Easter break the project of making quinine from aniline and serendipitously synthesized a blue purple dye which he first called Tyrian purple and later mauveine or mauvine. Also known as aniline purple, this became incredibly fashionable and caused a revolution in the history of dyes and color. It became all the rage with ladies of fashion, favorite color of empress Eugenie of France (leader of European fashion at the time) and adopted by Queen Victoria, who in 1862 at the Royal Exhibition made an appearance in a silk gown dyed with it. (figure2)

Perkins patented his serendipitous discovery. He set up a factory to produce mauveine and other aniline dyes. These became big business and gave rise to a new chemical industry, located mainly in Germany and supported by intensive research in universities. Many other new dyes and other substances were produced, such as methylene blue and violet, crystal violet, safranin, fuchsine, eosin, and hematoxylin, still household names and still widely used in laboratories now. And it was into this world of new aniline dyes that Paul Ehrlich was born in 1854 in a small town in Silesia.

While still a schoolboy, Ehrlich had become fascinated by these colorful dyes, largely influenced by his elder cousin, Carl Weigert, who owned one of the first microtome for cutting tissue into thin sections. At his home, Ehrlich would cut down the flowers in the garden and stain them with his dyes. Later in life, he became more interested in the molecular changes reflected by the different colors, or otherwise would have become a great artist, a Monet or Van Gogh, rather than a great scientist.

Fig. 4, Ehrlich’s reagent
Fig 3. The Charité in Berlin

He continued his interest during his medical student days at Breslau, Strasbourg, and Leipzig, and also later in 1878 when appointed assistant to the distinguished professor Theodor Frerichs at the Charité in Berlin (figure 3). He worked incessantly, spending most of his time in the laboratory, his fingers and hands often stained up to the wrist by aniline dyes. At the Charité he studied the cells of the blood by devising a new technique of fixing the specimen on a heated slide. Able with his dyes to differentially stain the nuclei, cytoplasm, and granules, he described mast cells, distinguished lymphocytes from neutrophils and eosinophils, and stained red and white blood cells and their precursors. He thus laid the basis for understanding the development of the red cell series and also for classifying the various forms of leukemia. He studied aplastic anemia, the megaloblasts of pernicious anemia, and the blood cells of paroxysmal nocturnal hemoglobinuria. He also developed a new test consisting of adding to urine a mixture of sulfanilic acid, nitric acid, and acidified sodium nitrite. This became known as Ehrlich’s reagent, widely used until recent times in the diagnosis liver diseases characterized by an increased excretion of urobilinogen (figure 4).

On a gray morning in March 1882, Ehrlich attended the lecture in which Robert Koch reported that he had discovered the organism causing tuberculosis.  This was a sensation, for Ehrlich his “greatest experience in science.” The next day in his laboratory he made a dry preparation in which bacteria fixed on a slide could be stained with methylene blue. It allowed the public for the first time to actually see the tuberculosis bacterium (figure 5).

Fig. 6, Paul Ehrlich
Fig. 5, Tuberculosis bacterium 

There was a big change in Ehrlich’s fortunes in 1885 when his chief, Professor Frerichs, committed suicide.  He was followed by Carl Gerhardt, who reorganized the department in such a manner that Ehrlich was assigned more clinical duties. Although his interests have always been basically in research, reports indicate that he carried his duties on the wards in an exemplary manner. In 1884 he had been appointed professor at the medical faculty of Berlin University and in 1887 he became a private lecturer there. However, his relations with Gerhardt deteriorated by 1887, and the next year he developed tuberculosis, which he diagnosed himself by examining his sputum with the technique he had pioneered. He spent two years in Egypt and southern Europe, recovered completely, and returning to Berlin in 1889 ran for two years a private laboratory and worked on an experimental chemotherapy by vital staining. This consisted of giving subjects small doses of methylene blue, a substance preferentially known to stain nerves. This had a mild analgesic action, leading Ehrlich to also try this approach in malaria, but with only moderate effects.

Meanwhile, the Prussian government had set up a new Institute for Infectious Diseases under the direction of Robert Koch, and in 1891 Ehrlich was invited to work there. In that Institute he carried out his first investigations on immunology, especially on hemolysins, the toxin-antitoxin reaction, and on antitoxic sera.  With von Behring, he developed a standardized serum to treat diphtheria and tetanus, but later the two had a falling out because Ehrlich did not receive his due share of profits when these agents were marketed, nor enough recognition when only von Behring (in 1910) received the Nobel Prize for that work.

Fig. 7, Creation of Salvarsan

In 1896 the Prussian government set up at Teglitz, a suburb of Berlin, an institute for the control of therapeutic sera.  Ehrlich became its director. Five years later, the Institute was moved to a pleasant flowering suburb in Frankfurt-am-Main, where Ehrlich was able to pursue his many research interests. There he ruled like an absolute monarch, smoking twenty-five expensive strong Havana cigars to stimulate but also calm his brain. It was in that setting that he developed his side-chain theory of how cells recognized friendly substances but opposed others by means of a chemical reaction, leading him to be awarded with Elie Metchnikoff the 1908 Nobel Prize for Physiology or Medicine. (figure 6)

Fig. 8, Ehrlichstrasse in Frankfort

He was also able to pursue his idea of a “magic bullet” that would destroy the invading organism without harming the host. Hundreds of substances were tested, among these an arsenic containing substance named atoxyl, which had been tried with some success to treat  sleeping sickness in animals. Then in 1903, when Schaudin and Hoffmann in Berlin discovered that syphilis was caused by a motile spirochete, Ehrlich decided to look again at the abandoned and considered worthless atoxyl. Working with Japanese research fellow Sahachiro Hata, Ehrlich was able to show that this compound was effective against the spirochete in the laboratory and also in human subjects. He named it Salvarsan, and very soon doctors from all over the world demanded to be given supplies of this agent. Manufactured by Hoechst AG, it was distributed widely and in 1911 was replaced by a less toxic substance, Neo-salvarsan.(figue7) These substances were the first effective treatments for syphilis and represented a huge advance in therapeutics.

After Ehrlich’s death in 1915, work on aniline compounds continued. It led to the development of agents to treat malaria and eventually to the sulfonamides, the first effective antibacterial agents before penicillin. Ehrlich received many awards and was made an honorary citizen of Frankfurt, where a tree-lined street was named after him.(figure 8) He has been figured on stamps and banknotes, and even had a crater on the moon named after him, as well as streets, schools, and institutions throughout the world. He is remembered as one of the great men of science, whose work has brought untold benefits to humankind.




GEORGE DUNEA, MD, Editor-in-Chief


Fall 2018  |  Hektorama  | Science