This section is from the book "A Text-Book Of Pharmacology, Therapeutics And Materia Medica", by T. Lauder Brunton. Also available from Amazon: A text-book of pharmacology, therapeutics and materia medica.
By careful cultivation through successive generations of a slip taken from a wild fruit-tree, the chemical processes of growth may be so modified in it that the fruit will lose its acrid character and become edible and pleasant. What is true of higher plants is true also of lower in this respect, and bacilli are much modified by the conditions under which they are cultivated; for example, Pasteur has found that the bacilli of anthrax develop and multiply in beef-tea best at 25°-40° C. Their development is retarded at lower or higher temperatures than these, and is completely arrested at 15° or 45° C. When cultivated at a temperature where development occurs with difficulty, such as 42°-43°, the bacilli no longer form resting spores, but only grow into long threads.
Fresh bacilli injected into an animal rapidly cause death from anthrax, but the longer they have been previously kept at this high temperature the more does their virulence decrease, and at the end of four or six weeks they die.
When some of the first crop of bacilli are put into fresh beef-tea, the second crop retains the degree of virulence of the first, and the third crop taken from the second, and again grown in fresh beef-tea, has exactly the same morbific power, and so on.
When the bacilli are cultivated at 35°, the microzymes not only multiply quickly, but they form spores of a definite degree of virulence, and these spores may be kept unaltered for years in sealed tubes, whereas the threads of developed bacilli die when air is excluded.
When an animal is inoculated with anthrax bacilli whose virulence has been diminished by cultivation at a high temperature, they produce merely temporary illness instead of death. By the growth of these non-virulent bacteria in the body, its constitution appears to undergo some alteration, and virulent bacteria subsequently injected have a much less powerful action on it. If the first injection be made with bacteria having a very slight amount of virulence, the animal may still die if injected a second time with virulent bacteria, but if inoculated first with non-virulent bacteria and a second time with bacteria rather more powerful, a slight disturbance is produced by each inoculation, and a subsequent injection of virulent bacteria no longer causes death. The changes which are produced by inoculation with modified anthrax or with vaccine matter in the blood and tissues, although probably very slight, are sufficient to confer on the organism immunity from further infection. This is usually permanent, although the immunity may diminish with the course of years, unless the advancing age of the animal in itself tends to lessen its liability to infection.
A similar immunity against infection with different bacilli is sometimes conferred by age. Thus young dogs are easily infected with anthrax, but old ones are not.
A difference of species also confers immunity. Thus rats and field-mice are not liable to infection with anthrax, while house-mice are highly so. Algerian sheep also resist infection with anthrax, while French sheep do not.
The experiments of Cash seem to show that it may be possible by the action of drugs to alter the blood and tissues in such a way as to render the animal proof against infection by pathogenic bacteria; for he has found that by the continued administration of minute doses of corrosive sublimate to animals he can render them capable of resisting the lethal effects of anthrax subsequently inoculated.1 This is a direction in which further research is likely to yield interesting results.