However, numerous researches on the prevention of cholera by means of divers microbes gave no results sufficiently conclusive to permit their application to human beings. The problem was rendered extremely complicated and difficult by the many and varied influences of numerous intestinal microbes and the inconstancy of microbian species in the same individual.
Pfeiffer's experiments, 1895--The Buda-Pest Congress--Extracellular destruction of microbes--Reaction of the organism against toxins-- Dr. Besredka's researches--Macrophages--The Moscow Congress, 1897-- Bordet's experiments.
Metchnikoff had scarcely recovered from all the emotions caused by his experiments on cholera, which he was still studying, when, in 1894, a work appeared by a well-known German scientist, Pfeiffer, bringing out new facts in favour of the extracellular destruction of microbes.
Whilst studying the influence of the blood serum within the organism and not outside it as his predecessors had done, he had found that cholera vibriones, injected into the peritoneum of a guinea-pig vaccinated against cholera, were nearly all killed in a few minutes and that they then presented the form of motionless granules in the peritoneal liquid. This granular degenescence, said Pfeiffer, took place apart from the phagocytes and therefore without their intervention. Metchnikoff repeated the experiment at once and ascertained that it was perfectly accurate.
The complexity of biological phenomena being very great, he fully admitted the possibility of other means of defence in the organism besides that of the phagocytic reaction. However, this new fact disagreed so much with his own observation, and seemed so isolated, that Metchnikoff supposed an error of interpretation must have been made and tried to throw light upon it. He spent sleepless nights seeking the conclusive experiment which might explain Pfeiffer's phenomenon.
His excitement was all the greater that he was very soon going to the International Congress at Buda-Pest, where he intended to expose the results of his new researches, and he feared that he should not have time to make all the experiments which he required in support of his arguments. However, the general impression of the Congress was clearly favourable to the phagocyte theory. This is how M. Roux picturesquely described the scene at Metchnikoff's Jubilee in 1915: