Louis Pasteur: His Life and Labours

by René Vallery-Radot

The virulence remained enfeebled and, we may say, unchanged. This showed it to be a real vaccine. Some veterinary surgeons and farmers, on the announcement of this discovery, applied to Pasteur for a vaccine against the disease which was so disastrous among their poultry. Some trials were made, and all succeeded beyond expectation. To preserve this vaccine it must be secured from contact with the air, the cultures being enclosed in tubes, the extremities of which are sealed by the flame of a blowpipe.

What takes place during that interval of time intentionally placed between two successive cultivations of the cholera microbe—that interval which is employed in effecting the attenuation and producing the vaccine? What is the secret of this result? The agent which intervenes is no other than the oxygen of the air. Here is the proof. If the cultivation of this microbe is carried on in a tube containing very little air, and if the tube is then closed by the flame of a lamp, the microbe, by its development and life, quickly appropriates all the free oxygen contained in the tube, as well as the oxygen dissolved in the liquid. Thus, completely protected from contact with oxygen, the microbe does not become sensibly weakened for months, sometimes even for years.

The oxygen of the air, then, appears to be the cause of modification in the virulence of the microbe.

But how, then, is the absence of influence on the part of the atmospheric oxygen, in the successive cultures which are practised every twenty-four hours, to be explained? There is, in Pasteur's opinion, but one possible explanation; it is that the oxygen of the air in this latter case is solely employed in the life of the microbe. A culture has a duration of some days; in twenty-four hours it is not terminated. The air which comes in contact with it is then entirely employed in nourishing and largely reproducing the microbe. During the longer intervals of culture, the air acts only as a modifier, and at last there arrives a moment when the virulence is so much weakened as to become nil.

This very extraordinary fact is, then, established that the virulence may be entirely gone while yet the microbe lives. The cultures offer the spectacle of a microbe indefinitely cultivable, yet, on the other hand, incapable of living in the bodies of fowls, and in consequence deprived of virulence. May not this domesticated microbe, as M. Bouley calls it, be compared to those inoffensive microbes of which there are so many in nature? May not our common microbes be those organisms which have lost their former virulence? But may not these harmless microbes, become infectious in some particular circumstances? And if there are fewer virulent maladies now than there were in times past, might not the number of these maladies again increase?

Questions multiply as the facts relating to the attenuation of a virus suggest inductions, awaken ideas, and throw new lights upon a problem which, until within these last few years, has remained so obscure. Formerly it was believed that these viruses were morbid entities. A virus was a unity. This opinion has still its declared upholders. According to Pasteur a virus has different degrees of virulence; it can pass from the weakest virulence to the maximum. Modifying, at will, the virus of fowl cholera, Pasteur inoculates some hens, for instance, with a virus too attenuated to protect from death, but which nevertheless is effectual in securing them against a virus stronger than itself. The second virus will preserve them from the attacks of a third virus, and thus passing from virus to virus they end by being guaranteed against the most deadly virulences. The whole question of vaccination resolves itself into knowing at what moment a certain degree of virus attenuation is a guarantee of protection against the mortal virus.

It seems that between small-pox and cow-pox facts of a similar kind take place. It is probable that vaccination rarely gives perfect security against the infection of a very malignant small-pox; moreover, during epidemics of small-pox many persons who have been previously vaccinated are attacked, and some even die of the disease.

As regards the practice of vaccinating fowls against the cholera peculiar to them—which, though it certainly is not of the same importance as human vaccination, is a scientifically capital fact—we may hope that whatever the differences of receptivity in different races, or in different individuals of the same race, there will be found vaccines to suit them all, special care being taken to resort to the employment of two successive vaccines of unequal power, employed after an interval of ten or fifteen days. The first vaccine may always be chosen of a degree of weakness which will not in any case cause death, and yet of sufficient strength to prevent dangerous consequences from the second vaccine, which would in some cases be fatal if employed at once, and to enable it to act as a vaccine against the most virulent virus.

With regard to the preparation of vaccines, and the ascertaining of their proper strength, it is necessary to make trials upon a certain number of fowls, even at the risk of sacrificing a few in these preliminary experiments. Beyond such questions of manipulation there remains still a scientific question. How are the effects of vaccination to be conceived? What explanation can be given of the fact that a benign disease can preserve from a more serious and deadly one? Pasteur long sought for the solution of this problem. Without flattering himself that he has unravelled the difficulty, he has nevertheless amassed facts which, amid these physiological mysteries, permit us to frame a hypothesis which can satisfy the mind. Pasteur believes, for example, that the vaccine, when cultivated in the body of the animal, robs the globules of the blood, for example, of certain material principles which the vital actions take a long time to restore to the system, and which to the most deadly contagium is a condition of life. The impossibility of action of the progressive virus and of the deadly virus is thus accounted for.

When Pasteur communicated to the Academy of Sciences these important and unforeseen facts, they were at first received with hesitation. It was not without some surprise that the word vaccination, hitherto exclusively reserved for Jenner's discovery, was heard applied to fowl cholera. At the International Medical Congress held in London in August 1881, Pasteur, in the presence of 3,000 doctors of medicine from all parts of the world, who received him with an enthusiasm which reflected glory on France, justified the name that he had given to his prophylactic experiments.