The next important step in this investigation was the discovery of the modification in the potency of the poison, which can be produced by the "cultivation" of this bacillus. Every one knows that some of our most valued esculent plants and fruits are the "cultured" varieties of types which man would scarcely care to use in their original state, on account of the unpleasantness of their flavor or their semi-poisonous qualities. And, now that we know that these disease-germs are really humble types of vegetation, the idea naturally suggests itself whether they, too, may not be so far modified, by the "environment" in the midst of which they are developed, as to undergo some analogous modification. Two modes of such "culture" suggest themselves: the introduction of the germs into the circulating current of animals of a different type, and its repeated transmission from one such animal to another; and cultivation carried on out of the living body, in fluids (such as blood-serum or meat-juice) which are found favorable to its growth, the temperature of the fluid in the latter case being kept up nearly to blood-heat. Both these methods have been used by Pasteur himself and by Professor Burdon-Sanderson; and the latter especially by M. Toussaint, of Toulouse, who, as well as Pasteur, has experimented also on another bacillus which he had found to be the disease germ of a malady termed "fowl-cholera," which proves very fatal among poultry in France and Switzerland. It has been by Pasteur that the conditions of the mitigation of the poison by culture have been most completely determined, so that the disease produced by the inoculation of his "cultivated" virus may be rendered so trivial as to be scarcely worth notice. His method consists in cultivating the bacillus in meat-juice or chicken broth, to which access of air is permitted while dust is excluded, and then allowing a certain time to elapse before it is made use of in inoculation experiments. If the period does not exceed two months, the potency of the bacillus seems but little diminished; but, if the interval be extended to three or four months, it is found that, though animals inoculated with the organism take the disease, they have it in a milder form, and a considerable proportion recover; while, if the time be still further prolonged, say to eight months, the disease produced by it is so mild as not to be at all serious, the inoculated animals speedily regaining perfect health and vigor.
- I have seen notices of its serious prevalence during this very summer in some of the localities most frequented by tourists.
- It is not a little curious that as culture of one kind can mitigate the action of the poison-germs, so culture of another kind may restore, or even increase, their original potency. It has been found by Pasteur that this may be effected by inoculating with the mitigated virus a new-born Guinea-pig, to which it will prove fatal; then using its blood for the inoculation of a somewhat older animal; and repeating this process several times. In this way a most powerful virus may be obtained at will a discovery not only practically valuable for experimental purposes, but of great scientific interest, as throwing light upon the mode in which mild types of other diseases may be converted into malignant.