Confronted with the hostility, indifference and scepticism which he found in the members of the Medical Academy, he once appealed to the students who sat on the seats open to the public.
“Young men, you who sit on those benches, and who are perhaps the hope of the medical future of the country, do not come here to seek the excitement of polemics, but come and learn Method.”
His method, as opposed to vague conceptions and a priori speculations, went on fortifying itself day by day. Artificial attenuation, that is, virus modified by the oxygen of air, which weakens and abates virulence; vaccination by the attenuated virus—those two immense steps in advance were announced by Pasteur at the end of 1880. But would the same process apply to the microbe of charbon? That was a great problem. The vaccine of chicken-cholera was easy to obtain; by leaving pure cultures to themselves for a time in contact with air, they soon lost their virulence. But the spores of charbon, very indifferent to atmospheric air, preserved an indefinitely prolonged virulence. After eight, ten or twelve years, spores found in the graves of victims of splenic fever were still in full virulent activity. It was therefore necessary to turn the difficulty by a culture process which would act on the filament-shaped bacteridium before the formation of spores. What may now be explained in a few words demanded long weeks of trials, tests and counter tests.
In neutralized chicken broth, the bacteridium can no longer be cultivated at a temperature of 45° C.; it can still be cultivated easily at a temperature of 42° C. or 43° C., but the spores do not develop.
“At that extreme temperature,” explains M. Chamberland, “the bacteridia yet live and reproduce themselves, but they never give any germs. Thenceforth, when trying the virulence of the phials after six, eight, ten or fifteen days, we have found exactly the same phenomena as for chicken-cholera. After eight days, for instance, our culture, which originally killed ten sheep out of ten, only kills four or five; after ten or twelve days it does not kill any; it merely communicates to animals a benignant malady which preserves them from the deadly form.
“A remarkable thing is that the bacteridia whose virulence has been attenuated may afterwards be cultivated in a temperature of 30° C. to 35° C., at which temperature they give germs presenting the same virulence as the filaments which formed them.”
Bouley, who was a witness of all these facts, said, in other words, that “if that attenuated and degenerated bacteridium is translated to a culture medium in a lower temperature, favourable to its activity, it becomes once again apt to produce spores. But those spores born of weakened bacteridia, will only produce bacteridia likewise weakened in their swarming faculties.”
Thus is obtained and enclosed in inalterable spores a vaccine ready to be sent to every part of the world to preserve animals by vaccination against splenic fever.
On the day when he became sure of this discovery, Pasteur, returning to his rooms from his laboratory, said to his family, with a deep emotion—“Nothing would have consoled me if this discovery, which my collaborators and I have made, had not been a French discovery.”
He desired to wait a little longer before proclaiming it. Yet the cause of the evil was revealed, the mode of propagation indicated, prophylaxis made easy; surely, enough had been achieved to move attentive minds to enthusiasm and to deserve the gratitude of sheep owners!