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If the sterilized leaves are placed in sterile water no fermentation occurs and no indigo is formed. If, however, some of the specific bacteria are added to the mass the fermentation soon begins and the blue colour of the indigo makes its appearance. It is plain, therefore, that indigo is a product of bacterial fermentation, and commonly due to a single definite species of bacterium.
The secretions which they produce are poisons to them as well as to the individual in which they grow, and after these have become quite abundant the further growth of the bacterium is checked and finally stopped. Partly, also, must we conclude that these hostile conditions are produced by active vital powers in the body of the individual attacked.
If any of the smaller species, especially Bacterium termo, are brought to rest, and then introduced into a fluid in which there is the minutest trace of free oxygen, they will immediately begin to move about freely; and if the oxygen is gradually introduced, their motion will be set up only in those parts of the drop which the oxygen reaches.
These we may briefly consider under two heads, viz., variation in the bacterium, and variation in the susceptibility of the individual. The first will require only a brief consideration. That the same species of pathogenic bacteria at different times varies in its powers to produce disease has long been known. Various conditions are known to affect thus the virulence of bacteria.
He stood near the entrance of the house, writing down in a note-book a thought that occurred to him. "If," he wrote, "a bacterium were to observe and analyze the nail of a man, it would declare him an inorganic being. Similarly, from an observation of the earth's surface, we declare it to be inorganic. That is wrong."
To do this it is necessary to find some particular bacterium present in all cases of the disease; to find some method of getting it to grow outside the body in culture media; to demonstrate its absence in healthy animals, or healthy human individuals if it be a human disease; and, finally, to reproduce the disease in healthy animals by inoculating them with the bacterium.
The knowledge that the disease was due to a living bacterium, that without the germ and the spread of the germ the plague could not exist, was a virtual announcement that in the civilized countries it should not any longer exist. The discoverer was now set high in the estimation of mankind. Imperial Germany best of all countries rewards its benefactors.
After souring, therefore, the milk for some time does not ordinarily undergo any further changes. Milk souring has been commonly regarded as a single phenomenon, alike in all cases. This bacterium has certainly the power of souring milk rapidly, and is found to be very common in dairies in Europe.
The presence of this acid is occasionally troublesome, since it is one of the factors in the rancidity of butter and other similar materials. The preparation of indigo from the indigo plant is a fermentative process brought about by a specific bacterium. The leaves of the plant are immersed in water in a large vat, and a rapid fermentation arises.
"You see, that man came to my house to see me, and he is an Anarchist. No don't faint, or I cannot possibly tell you the rest. And I wanted to astonish him, not knowing he was an Anarchist, and took up a cultivation of that new species of Bacterium I was telling you of that infest, and I think cause, the blue patches upon various monkeys; and, like a fool, I said it was Asiatic cholera.
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