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HAVING ascertained that infection results from
the introduction of living “disease germs ” into susceptible individuals, it will be well to give some consideration to these agents of infection. The term disease germ is a popular one and is used to designate any micro-organism capable of giving rise to an infectious disease. The word micro-organism, which I shall have frequent occasion to use, may require a little explanation. By an organism we mean an organic structure which has been built up by vital processes. It may be a plant or an animal, it may be complex or simple, large or small, but it must, at one time at least, have been endowed with life. A microorganism is simply a microscopic organism, and being microscopic it is an organism of very simple structure, usually consisting of a single cell (“unicellular microorganism"). When using the word micro-organism with reference to a disease germ, we must use an adjective to indicate that the particular micro-organism under consideration is capable of producing disease, for there are numerous micro-organisms which are entirely harmless. The word pathogenic literally means disease-producing; a pathogenic micro-organism is therefore a microscopic organism capable of producing disease. This sounds a little more scientific than “disease germ," and it has seemed to me necessary to spend a little time in explaining the meaning of the term, as I may have occasion to use it from time to time, although it is my intention to avoid the use of technical terms as far as possible.
When we say that a certain infectious disease is due to a pathogenic micro-organism we have not committed ourselves as to the characters of this disease germ. It may belong to the animal or the vegetable kingdom ; it may be round or oval or spiral in form; it may be large or small, although microscopic. But when I speak of the micrococcus of pneumonia or the bacillus of typhoid fever, I am using terms which convey much more definite information with reference to the disease germs referred to, and before proceeding any farther it will be desirable to make the reader acquainted with the principal characters of some of the best-known pathogenic micro-organisms. Some of these belong to the animal and some to the vegetable kingdom. Although so very minute and simple in structure, consisting, as a rule, of a single cell, they may be differentiated by the expert without great difficulty, and classified as animal micro-organisms (“Protozoa") or as vegetable micro-organisms (“Protophyta "). By far the greater number of known disease germs are recognised as vegetable micro-organisms belonging to the class known as Bacteria. This class includes a large number of harmless species, which abound especially in surface waters and in the upper layers of the soil.
The bacteria are classified with reference to their form. Those which are spherical are called micrococci; those which are longer in one diameter than in the other-oval, rod-shaped, or filamentous—are called bacilli ; those which are elongated and spiral in form, like a corkscrew, are called spirilla (singular-micrococcus, bacillus, spirillum). The germs of pneumonia, of erysipelas, of boils and abscesses, of Malta fever, of cerebro-spinal meningitis, and some others are micrococci, all having distinct specific characters by which they can readily be recognised by an expert bacteriologist. The germs of typhoid fever, of tuberculosis, of influenza, of diphtheria, of dysentery, of bubonic plague, of tetanus, and of several infectious diseases of the lower animals (hog cholera, swine plague, anthrax, glanders) are bacilli. As in the case of the pathogenic micrococci, these all have specific characters by which they can be differentiated one from the other, independently of the fact that each gives rise to a specific infectious disease. The germs of Asiatic cholera and of relapsing fever are spirilla.
All bacteria multiply by binary division—that is, one cell divides into two, each resembling in form and dimensions the parent cell, and each in its turn dividing in the same way. The rapidity of multiplication by binary division varies greatly in different species, and depends upon circumstances relating to temperature, moisture, and suitable nutrient material. Under favourable conditions bacilli have been observed to divide in twenty minutes, and, as each daughter cell is equal in size to the mother cell, it is evident that an amount of nutrient material has been assimilated during this time equal to the quantity contained in the original cell. As a result of this rapid development, “colonies" containing millions of bacilli may be developed from a single cell in twentyfour to forty-eight hours. A simple calculation will show what an immense number of cells may be produced in this time as a result of binary division occurring, for example, every hour. The progeny of a single cell would be at the end of twenty-four hours 16,777,220. During the process of multiplication by binary division, the bacterial cells often remain attached to each other, and we may see them under
the microscope grouped in pairs, or in chains, or in irregular masses.
Some of the bacteria multiply not only by binary division but also by the formation of spores, which correspond, so far as the preservation of the species is concerned, to the seeds of higher plants. The growing cells are delicate plants which are easily killed by heat and by various chemical agents (disinfectants). But the spores resist a much stronger solution of germicidal agents and a much higher temperature. They also resist desiccation, and may retain their vitality for months or years until circumstances are favourable for their development, when, under the influence of heat and moisture, they reproduce the minute microscopic plant-bacillus or spirillum—and multiplication by binary division again occurs. It is fortunate that comparatively few pathogenic bacteria produce spores, for if this were the case it would be a much more difficult task to arrest the progress of an epidemic of such diseases as typhoid fever, bubonic plague, cholera, or diphtheria. The only infectious disease of man in which spores have been demonstrated to be forined is tetanus, or lockjaw. As this disease is not likely to be communicated by the sick to those associated with them, either directly or indirectly, the formation of spores by the tetanus bacillus is not so serious a matter.