As he failed to solve the problem of the specificity of the cholera vibrio on animals, he resolved to experiment upon himself and consumed a culture of cholera vibriones. He did not contract cholera, which made him doubt the specificity of the vibrio, and therefore he consented to repeat the experiment on one of his workers (M. Latapie) who offered to submit to it: the result was the same. He then did not hesitate to accept the offer of a second volunteer (M. Jupille). The preceding results having led him to suppose that the cholera vibrio became attenuated _in vitro_ and might perhaps serve as a vaccine against cholera, he gave a culture of long standing to the young volunteer.
To his astonishment and despair, Jupille began to manifest the typical symptoms of cholera, and a doctor who was particularly conversant with the clinical chart of the disease declared the case a severe one because of the nervous symptoms which accompanied it.
Metchnikoff was in mortal anxiety, and even said to himself that he could not survive a fatal issue. Fortunately the patient recovered, and this terrifying experiment proved indisputably the specificity of the cholera vibrio. Yet the irregularity of its action showed that in certain cases conditions existed which prevented the inception of the disease, and Metchnikoff supposed that this might be due to the action of the different intestinal micro-organisms.
In order to simplify the question, he began by making experiments outside the organism. He sowed the cholera vibrio with divers other microbes and saw that some of them facilitated its culture whilst others prevented it. Similar experiments within the organism of animals gave no conclusive results; the simultaneous ingestion of the cholera vibrio and of favourable microbes did not induce cholera.
The flora of the intestines, complex as it is, probably played a part on which it was difficult to throw any light. Yet Metchnikoff did not give up the idea of producing a vaccine against this disease with attenuated microbes, or, if not, to prevent its inception by preventive microbes. His thesis was strengthened when one of his pupils, Dr. Sanarelli, discovered a series of choleriform bacilli in the absence of any cholera epidemic, one of those microbes being found at Versailles, a town which had remained immune during every cholera epidemic.