Culture of' Organisms. — What is the general method adopted in investigating the nature of micro-organisms? By the use of very highly magnifying powers, and by the use of staining agents, they can be seen under the microscope. In the blood of an animal dead of splenic fever an organism was discovered by means of the microscope, hut no information could thus be gained as to its relationship to the disease. Often, moreover, various kinds of such living things were found, and the question arose which kind, if any, was it that produced the disease.
Experimenters, therefore, attempted to grow artificially the different organisms. Some flourish and multiply in chicken-broth, others grow well on gelatine, some on raw potatoes, while various other solids and fluids were found capable of nourishing them. Experi ment showed, moreover, that a particular form of organism flourished so well in one particular fluid that, though thiakind with many others were put into a flask of the fluid, the particular form took entire possession of the fluid, and no other had a chance of life against it. Here, then, was a method of sifting out one kind from another, until a fluid containing one form only was obtained with which to experiment. Thus a drop of blood taken from an animal dead of splenic fever is placed in a flask containing meat-broth, which has been shown to be free of germs of all kinds by having stood for several days or weeks, after being boiled, without the least trace of decomposition occurring in it. The flask is plugged with wool, and is main tained at a certain heat. In a few hours the bacilli of splenic fever, present in the drop of blood, have multiplied enormously. If other kinds of organisms were present in the drop they may have grown too, but to a less extent, because the soil, so to speak, is not so suitable for them. From this flask a drop of fluid is taken and transferred to a second flask con taining the same pure broth free of germs. The special bacillus multiplies here again, other forms less. From the second flask a drop is transferred to a third, and so on through six or seven flasks, till a fluid is obtained' contain ing the one particular organism only, all the others having died out. This flask DOW con tains what is called a pure cultivation. Now if the bacillus contained in this fluid is the cause of splenic fever, then the injection of a small quantity of the fluid into an animal, cap able of taking the disease, ought to produce the disease in the animal. If it produces the
disease, then in the blood of the animal the same bacillus should be found, and from a drop of the blood new quantities of the or ganism ought to be capable of being reared, by means of which, in turn, the disease can be again communicated. All these different pro cesses must be gone through before it can be said with perfect certainty that the particular germ is the active cause of the particular dis ease. Besides all this, it is plain that when a cultivation of the germ is obtained, experi ments may be performed to determine what substances hinder and what aid its growth, whether carbolic acid, Condy's fluid, or other agents kill it, and so on. By such means in formation may be gained that would enable the disease to be arrested or stamped out.
A remarkable illustration of this may be given from Pasteur's work. He found that hens never took splenic fever, and that the disease could not in an ordinary way be com municated to them. Now a degree of beat equal to Centigrade kills the splenic fever bacillus, and the heat of hen's blood is 41° or 42°. He thought perhaps the high temperature of the fowl's blood prevented it from taking the disease. So he took a hen and placed it in a cold chamber till its bodily heat was lowered to 37°. He then injected the poison; it took the disease and died. He did the same with another fowl; but this time, at the height of the attack, he placed it in a warm chamber to raise its bodily heat up to or above the usual. It recovered. But, again, Pasteur found that an animal that had recovered from one attack of splenic fever was safe from a second attack. He found that he could cultivate the splenic bacillus through hundreds of generations with out its violence being the least affected, pro vided one cultivation followed another within an interval of hours. If, however, a cultivation of the organism were left for days or months with a due supply of pure air, its violence was remarkably diminished, and if this weakened bacillus were infected into an animal as animal was very slightly affected for a shOrt time, but was rendered incapable of acquiring the fatal form of the disease.