As the discoverer of the specific causative agents of deadly infectious diseases including tuberculosis, cholera and anthrax, he is regarded as one of the main founders of modern bacteriology.

During his fifth semester at the medical school, Jacob Henle, an anatomist who had published a theory of contagion in 1840, asked him to participate in his research project on uterine nerve structure.

[13] In his sixth semester, Koch began to research at the Physiological Institute, where he studied the secretion of succinic acid, which is a signalling molecule that is also involved in the metabolism of the mitochondria.

[18][19] Robert Koch is widely known for his work with anthrax, discovering the causative agent of the fatal disease to be Bacillus anthracis.

[20] After officially becoming a district physician in Wollstein (today's Wolsztyn), Poland, in 1872, Robert began to delve into the disease called Anthrax.

By gaining a better understanding of this pathogen, he was able to shed light on the bacterium's remarkable resistance to environmental factors ("Robert Koch – Nobel Lecture" 2018).

[31] Robert Koch made two important developments in microscopy; he was the first to use an oil immersion lens and a condenser that enabled smaller objects to be seen.

[25][33] Koch's booklet published in 1881 titled "Zur Untersuchung von Pathogenen Organismen" (Methods for the Study of Pathogenic Organisms)[34] has been known as the "Bible of Bacteriology.

[25] He published the discovery as "Die Ätiologie der Tuberkulose" (The Etiology of Tuberculosis),[33] and presented before the German Physiological Society at Berlin on 24 March 1882.

Koch said,When the cover-glasses were exposed to this staining fluid [methylene blue mixed with potassium hydroxide] for 24 hours, very fine rod-like forms became apparent in the tubercular mass for the first time, having, as further observations showed, the power of multiplication and of spore formation and hence belonging to the same group of organisms as the anthrax bacillus...

Microscopic examination then showed that only the previously blue-stained cell nuclei and detritus became brown, while the tubercle bacilli remained a beautiful blue.

"[5] Koch expanded the report and published it under the same title as a booklet in 1884, in which he concluded that the discovery of tuberculosis bacterium fulfilled the three principles, eventually known as Koch's postulates, which were formulated by his assistant Friedrich Loeffler in 1883, saying:All these factors together allow me to conclude that the bacilli present in the tuberculous lesions do not only accompany tuberculosis, but rather cause it.

[43]In August 1883, the German government sent a medical team led by Koch to Alexandria, Egypt, to investigate a cholera epidemic there.

[44] Koch soon found that the intestinal mucosa of people who died of cholera always had bacterial infection, yet could not confirm whether the bacteria were the causative pathogens.

Koch's colleague Richard Friedrich Johannes Pfeiffer correctly identified the comma bacillus as Pacini's vibrioni and renamed it as Vibrio cholera in 1896.

He developed a method for examining antibacterial activity by mixing the gelatin-based culture media with a yellow dye, auramin.

Based on a series of experiments from April to July 1891, he could conclude that the extract did not kill the tuberculosis bacterium, but destroyed (by necrosis) the infected tissues, thereby depriving bacterial growth.

He made a vague announcement in August 1890 at the Tenth International Medical Congress in Berlin,[43] saying,In a communication which I made a few months ago to the International Medical Congress [in London in 1881], I described a substance of which the result is to make laboratory animals insensitive to inoculation of tubercle bacilli, and in the case of already infected animals, to bring the tuberculous process to a halt.

[5]By November 1890, Koch demonstrated the effectiveness of the extract in treating humans by administering the vaccine through the Bacillus Calmette-Guerin (BCG) technique.

[13] While this was effective in humans, his experiments also revealed that when the substance was inoculated into his tuberculosis-infected test guinea pigs, they presented with severe symptoms.

When a normal guinea pig was inoculated with pure tubercle bacillus, the wound would close rapidly and heal within several days.

[63] Koch's discovery of the causative agent of anthrax led to the formation of a generic set of postulates which can be used in the determination of the cause of most infectious diseases.

[23] On 27 May, three days after giving a lecture on his tuberculosis research at the Prussian Academy of Sciences, Koch died in Baden-Baden at the age of 66.

[69] Koch was made a Knight Grand Cross in the Prussian Order of the Red Eagle on 19 November 1890,[71] and was elected a Foreign Member of the Royal Society (ForMemRS) in 1897.

"[73] In 1906, research on tuberculosis and tropical diseases won him the Order Pour le Merite and in 1908, the Robert Koch Medal, established to honour the greatest living physicians.

His life was the subject of a 1939 German-produced motion picture that featured Oscar-winning actor Emil Jannings in the title role.

[82] In 1898, an American veterinarian Theobald Smith published a detailed comparative study and found that the tuberculosis bacteria are different based on their structure, growth patterns, and pathogenicity.

In his conclusion, he made two important points: By that time, there was evidence that cattle tuberculosis was transmitted to humans through meat and milk.

But it could not make the final decision on whom to give it to — the British surgeon Ronald Ross or the Italian biologist Giovanni Battista Grassi.

[91] Ross was selected for the award, as Koch "threw the full weight of his considerable authority in insisting that Grassi did not deserve the honour".