CHAPTER XXII. EXPERIMENTAL METHODS FOR TESTING THE VALUE OF A (231) Distinction between Disinfectants and Antiseptics. A TRUE disinfectant is a chemical substance which is capable of killing by its poisonous action a pathogenic, or disease-producing germ; it is therefore synonymous with "germicide." Disinfection is to be distinguished from "destruction," as, for instance, by fire or by the corrosive action of strong mineral acids; a genuine disinfectant does not necessarily change the structure of the tissues or bacteria submitted to its influence. Antiseptics are substances that arrest the development of germlife; all disinfectants are also antiseptics, if employed in quantities insufficient to destroy life, but antiseptics are not necessarily germicidal. The elements modifying disinfection are quantity of disinfectant in relation to the thing to be disinfected, temperature and time. In other words, the disinfectant must be of sufficient strengthmust act on the substance to be disinfected for a sufficient period of time. (232) Temperature. Temperature is important, for the writer has proved that, e.g. 115 per cent. of lutidine, acting for twenty-four hours at 15°, failed to disinfect a thread infected with bacterium termo, but 5 per cent. at 35°.5 of lutidine disinfected absolutely. The element of time is all important; nevertheless, the experiments of Cash have proved that a disinfectant in a too dilute state may act for a lengthened time on pathogenes without any effect, but if the disinfectant be of such a strength that it is capable of disinfecting at all, the longer the time ceteris paribus, the more complete the disinfection. (233) Experimental Methods. There are two methods by which a chemical agent may be tested:-First of all, to find out whether it is a disinfectant at all; and secondly, if it should be a disinfectant, what dose or strength should be used? 1. Inoculation into Animals of Disinfected Matter.-The virus of a disease, or a pure cultivation of a pathogene, is taken, such as tuberculosis, mouse septicæmia, anthrax, submitted to the action of a disinfectant of a definite strength for a definite time and temperature, and then the product is injected into an animal known to be susceptible. At the same time a similar quantity of the undisinfected virus is injected into another animal, called "the control,” and the effects noted. This method of experiment is by far the most satisfactory and conclusive. 2. Testing "Cultures.”—The second method is of great utility, because it enables an observer to arrange disinfectants in their order of strength, and essentially consists in submitting cultivations. of microbes to the action of disinfectants, and then ascertaining their power of development in nutrient soils. The writer has employed certain processes for this purpose, which he has thus described (“Studies of Disinfectants by New Methods,” Proceedings of the Royal Society, 1886). (234) The Drop Method. Sterilised pure water is infected with a few c.c. of gelatin liquefied by the bacterium; measured volumes of this infected water are then added to measured volumes of the disinfectant, and the whole allowed to act for a definite time. A drop of this liquid is then added to from 10-20 grams of the nutrient gelatin, first liquefying it at a very gentle heat. As the proportion of the weight of the drop to the weight of the nutrient gelatin varies from about 1:500 to 1:1000, the dilution is in most cases sufficient to reduce any antiseptic or inhibitory action of the minute quantity of the chemical agent in the drop itself to a minimum, so as to exercise no appreciable effect. |