Sechster Bericht der deutschen wissenschaftlichen Commission zur Ehrforschung der Cholera.

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.)