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he asserted that the organisms of fermentation were analogous to those described by Davaine in anthrax. Klebs in 1871 furnished the direct proof of the bacterial origin of disease by his studies on wound infections; and later researches have amply confirmed the microbic nature of infectious diseases.

But how do the microbes produce the lesions and symptoms of disease? Micro-organisms, although multiplying in the tissues of the host, are usually too insignificant in size to produce any appreciable result by their mere numbers. It appears, however, that this mechanical rôle is played in anthrax (as also in other septicæmic infections), where organisms fill up the lumen of capillaries, and thereby derange the normal metabolism of the affected parts. But in most cases the phenomena of disease are produced by the secretion of toxins or poisonous products, which enter into combination with the constituents of the body cells. In fact, it may generally be said that the bacterial infection is of the nature of intoxication. For although the multiplication of the organisms in certain tissues alimentary canal, tonsils, etc.) is necessary to regulate the supply of toxin, the specific symptoms of the disease are produced by the latter alone. This can be easily demonstrated by injecting a germfree culture, say, of the diphtheria bacillus (obtained by passing a broth culture of this organism through a Chamberland filter), into a susceptible animal, when the characteristic paralytic phenomena of the disease are instantly reproduced. But the local inflammatory lesions cannot be produced in this manner, which shows that some toxins are manufactured in the living tissues alone. As a matter of fact, pathogenic germs form a series of substances of varying degrees of toxicity, and thereby contribute to the complexity of the symptoms of infectious diseases. Thus,

the secondary anæmias which occur in the course of pneumonia, typhoid, etc., are due to the destructive action of certain of their toxins on the red-blood corpuscles of the host. But, although the bacterial products are of a complex nature, the specific symptoms are always produced by specific poisons.

The toxins may be soluble (extracellular), as in B. diphtheria and B. tetani ; or they may be insoluble and contained in the bacterial cell (intracellular), as in the cholera and typhoid organisms. The intracellular toxins, also called “bacterioproteins," are common to many bacteria ; and when injected into animals produce fever and inflammatory symptoms. According to Buchner, there is no hard and fast line between the intra- and extracellular toxins; and he regards both of them as closely associated with the cell protoplasm. For, just as in alcoholic fermentation, the yeast cells themselves contain alcohol, it may be that the toxins are first produced in the bacterial cells, and subsequently excreted into the surrounding medium.

It is, however, impossible to say whether the extracellular toxins are actually excreted, or are produced by the bacteria acting on the chemistry of the nutrient medium. Toxins have not been isolated in the pure state, and their precise nature is not known. Roux regards them as enzymes. According to most authori. ties, however, they are either proteids, or linked on to proteid molecules, and are not unlike certain vegetable and animal poisons, as ricin, abrin, snake venom, etc. All these bodies are sometimes called toxalbumins.

The lesions produced in disease may be situated in the vicinity of bacteria, or at a distance. The former are

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into groups.

caused by the direct action of the toxins; and the latter by their absorption into the systemic circulation. The structural changes thus induced cause various functional disturbances, the so-called clinical symptoms.

It must be clearly understood that what is called a “ disease is not a specific entity, but a series of morbid manifestations, which, for convenience, have been arranged

The classification of diseases was made long before the advent of bacteriology; and thus it may or may not correspond with the division based upon the germ theory. Pneumonia, for instance, is a distinct disease, and yet it may be caused by a variety of organisms. Again we know that the organism of pneumonia, besides producing this effect, may also give rise to ædema, suppuration or septicemia.

On the other hand, many diseases are due to a single organism, which may truly be called specific. Anthrax, for example, is always caused by the B. anthracis; and this organism produces no other disease besides anthrax. Diphtheria is another notable example of a specific disease; but the same cannot be said of septicæmia or pneumonia. Koch has laid down that every specific pathogenic germ must fulfil the following conditions : (1) It must be constantly present in the diseased

tissues, and must have special relations with

the tissue changes. (2) It must be capable of being cultivated on artificial

media. (3) The pure cultures thus obtained must reproduce

the disease when inoculated into fresh animals. The researches of the last few years have brought out the interesting fact that every pathogenic germ is really a member of a family of organisms possessing closely allied characters. This discovery does not invalidate the

doctrine of the specificity of disease, but is interesting from the evolutionary point of view, for it serves to remind us that all our parasites have arisen from saprophytes, by the process of natural selection. The organisms which simulate pathogenic forms are usually called

pseudo-bacteria. This term, however, is apt to be misleading, and will only exceptionally be used in the following pages.

SUPPURATION.

But pus,

EXPERIMENTALLY, suppuration may be produced by various chemical means, as, for example, by the subcutaneous injection of silver nitrate or oil of turpentine. as met with clinically, is usually formed by the action of certain bacteria, the most important of which are the staphylococcus pyogenes aureus and the streptococcus pyogenes. Several other varieties are also commonly present, as, for instance, staphylococcus pyogenes albus and citreus, and bacillus pyocyaneus (the organism of green pus). It may also be mentioned that the organisms of gonorrhoea, pneumonia, anthrax, etc., are frequently concerned in

pus

formation. Staphylococcus Pyogenes Aureus.--This organism, which is normally present on the skin and in the respiratory mucous membrane, is the most common cause of suppurative processes in general. It can be obtained almost in pure cultures from boils, carbuncles, and the pus of osteomyelitis.

Under the microscope the organisms are observed as spherical cells forming groups or clusters of various sizes. They multiply rapidly at 20° C. in milk, broth, and other nutrient media. In “stab cultures”I the gelatine is liquefied in the form of a pouch, the bottom of which contains

For methods of cultivation, staining, etc., see Appendix A,

p. 129.

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