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Perhaps one of the most common troubles due to bacterial infection of eggs is the more or less serious illness sometimes caused by eating those which are 'stale. This often resembles ptomaine poisoning, which is caused, not by micro-organisms themselves, but by the poisonous products which they elaborate from materials on which they grow.
The various cells are attracted to the bacteria by a peculiar chemical or biological power known as chemotaxis, which seems to result from variations in the surface tension of different varieties of cells, probably caused by some substance produced by the micro-organisms.
It will thus be seen that while some microbes are undesirable, others exert a beneficial action. It is only within comparatively recent years that the action of micro-organisms has been understood. It is now definitely known that these minute living things seize every possible chance to attack articles of food and produce the changes known as fermentation, putrefaction, souring, and decay.
It is like the compositor distributing his type after the book is printed. The micro-organisms answer to the compositor, but they are of a different kind from those which build up the body in the first instance. But the living body as a whole, with its complex of coördinating organs and functions what attended to that? The cells build the parts, but what builds the whole?
At the outset the tobacco producer has to contend with a number of micro-organisms which may produce diseases in his tobacco. During the drying process, if the temperature or the amount of moisture or the access of air is not kept in a proper condition, various troubles arise and various diseases make their appearance, which either injure or ruin the value of the product.
Many diseases, such as diarrhoea, enteric fever, and cholera, and perhaps tuberculosis, may be caused by eating infected food. Trichiniasis may also be mentioned. Tinned fish often gives rise to symptoms of poisoning, and shell-fish are not uncommonly contaminated with pathogenic micro-organisms.
So thoroughly is this true that, as we shall find after a short consideration, the continuance of life upon the surface of the world would be impossible if bacterial action were checked for any considerable length of time. The life of the globe is, in short, dependent upon these micro-organisms.
And so it came as a new revelation to the generality of scientists of the time, when, in 1857 and the succeeding half-decade, Pasteur published the results of his researches, in which the question had been put to a series of altogether new tests, and brought to unequivocal demonstration. He proved that the micro-organisms do all that his most imaginative predecessors had suspected, and more.
There was nothing in these studies bearing directly upon the question of animal diseases, yet before they were finished they had stimulated progress in more than one field of pathology. At the very outset they sufficed to start afresh the inquiry as to the role played by micro-organisms in disease.
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