In 1876 Koch first obtained pure cultures of B. anthracis and described its complete life history. With Davaine (Nos. 5165-66) he did much to prove that infectious diseases are caused by living reproductive microorganisms. The paper also marks the beginning of exact knowledge of bacterial infectious diseases. It is reproduced with translation in Med. Classics, 1938, 2, 745-820. See also Nos. 2331 and 2536.

Koch greatly improved staining methods; he laid the foundations of the technical procedures employed in bacteriology today. In the above paper he described his method of slide preparation, making films of bacteria on cover-slips and fixing them gently by heat, his methods of staining, and preserving the specimens. He also gave details of his method of photographing bacteria, and reproduced the first photomicrographs of bacteria. In some of his reproductions of photomicrographs the cilia are clearly perceptible.

Koch’s epochal work on the etiology of traumatic infectious disease established his reputation. He inoculated animals with material from various sources and produced six types of infection, each due to microorganisms. He carried these infections through several generations of animals. These experiments determined the role of bacteria in the etiology of wound infections and demonstrated for the first time the specificity of infection. This work also contains the first explicit statement of the criteria implicit in Henle (See No. 2533) on contagion, which later became known as Koch’s postulates. See also Nos. 2331 and 5167. English translation, New Sydenham Society, 1880.

Koch’s description of his methods of growing bacterial cultures in gelatine solutions, making films of bacteria on cover slips and fixing them by gentle heat, and staining slides differentially by aniline. These methods are the bases on which bacteriology largely rests.

Koch showed that mercuric chloride was superior to carbolic acid, and that live steam surpassed hot air in sterilizing power.

Discovery of the tubercle bacillus announced March 24, 1882. This paper also contains a statement of “Koch’s postulates”. See also Nos. 2536 and 5167. Koch published a fuller account as "Die Aetiologie der Tuberkulose," Mitt. k. Gesundh. Amte,  2 (1884) 1-88, in which he reported how he had succeeded in producing experimental tuberculosis in animals after cultivating the bacillus. Historian of bacteriology Thomas Brock stated that the 1884 paper "announced what became known as Koch's postulates." Reprinted with translation in Med. Classics, 1938, 2, 821-80. 

In 1905 Koch was awarded the Nobel Prize in Physiology or Medicine "for his discoveries and investigations related to tuberculosis."

In 2019 Juan Weiss pointed out that on p. 225 of this work Koch published the first reference to the discovery of Agar, without crediting its discoverer, Walther Hesse, an assistant who worked in Koch's laboratory at the time.

Koch–Weeks bacillus, Hemophilus conjunctivitidis, or Hemophilus aegyptius. Koch discovered the bacilli of two varieties of Egyptian conjunctivitis. See also No. 5930.

In this paper, dated 2 February 1884, written while Koch and his team were in Calcutta, Koch first published his discovery of the cholera bacillus and the main route of its transmission. He based his discovery on the unique microscopic morphology of the bacillus and its motility in gelatin, and liquefaction of that gelatin by this novel bacillus. He then reinforced the discovery by including autopsy evidence of deep invasion of the intestinal tissue by this bacillus.

Koch followed this paper with a second paper dated 4 March 1884, also from Calcutta:
Bericht des Leiters der deutschen wissenschaflichen commission zur Erfoschung der cholera. Dtsch. med. Woch. 10, No. 14, 1884, 221-222.

In this follow-up paper Koch and colleagues proved the transmission of the cholera bacillus through water. They isolated the bacillus with the same bacteriological characteristic described in their 2 February 1884 paper from a specific tank supplying water to some individuals. Koch's team then observed natural transmission specificaly and only to the individuals who drank this water, and subsequent development of characteristic cholera illness in those individuals.

(Thanks to Juan Weiss for these references and their interpretation.)

In 1890, Koch announced the discovery of tuberculin, a substance derived from tubercle bacilli, which he thought was capable of arresting bacterial development in-vitro and in animals. This news gave rise to tremendous hope throughout the world, which was soon replaced by disillusionment when the product turned out to be an ineffective therapeutic agent. In this paper Koch provided his definitive expression of "Koch's Postulates."

The second paper described “Koch’s phenomenon”, and tuberculin skin test. Koch showed that tuberculin injected intradermally would elicit a severe local inflammatory reaction in tuberculous patients. This was the first diagnostic skin test. Abbreviated English translation of second paper in Bibel, Milestones of immunology (1988).

Koch’s new tuberculin (Tuberculin R).

The prophylactic measures for the control of typhus suggested by Koch have been adopted almost everywhere.

For his work on tuberculosis Koch received the Nobel Prize in 1905. Digital facsimile from the Internet Archive at this link. See T.D. Brock, Robert Koch: A life in medicine and bacteriology, Madison, WS: Science-Tech Publishers, 1988.