Germ, a name applied to the egg-cell of plant or animal, either from the first or in its early stages; but also used in reference to micro-organisms associated with disease (see BACTERIA, &c.). By 'germ-cells' the reproductive elements, especially the ova, are meant; while 'germ-plasma' is a very common modern word for the most essential parts of the nuclei in the reproductive cells. See EMBRYOLOGY, HEREDITY.
GERM THEORY OF DISEASE, as the name implies, seeks to find the explanation of certain well-recognised conditions of disease in the presence and action of specific living organisms within the affected body. Though comparatively recently introduced as an efficient working hypothesis in the investigation of some hitherto ill-understood pathological phenomena, the correctness of the theory is now generally admitted. The facts which it has aided in establishing and the numberless investigations which it has inspired have created an important department of medical science. The study of bacteriology (see BACTERIA) has awakened fresh interest in almost every branch of medicine; and the subject possesses a large and extensive literature of its own.
The evolution of the theory was due mainly to two factors: (1) The discussions and investigations which circled round the process of fermentation; (2) the application of more perfect microscopical methods to the study of the lowest forms of plant and animal life.
(1) The familiar process of Fermentation (q.v.) gave birth to much debate. The earlier chemists (Gay-Lussac, and more recently Liebig) held that fermentation was merely the result of the process of decay of organic matter. Various modifications of this doctrine, which cannot be considered here, were enunciated, but the general conclusion remained the same. On the other hand, so early as 1812, Appert had demonstrated from the practical side that organic substances capable of fermentation or putrefaction could be preserved intact if kept in closely stoppered bottles which were afterwards exposed to the temperature of boiling water. In 1836 Cagniard-Latour described an organism, the yeast plant, which he affirmed to be constantly present in the fermenting fluid. Its growth and reproduction he believed to proceed synchronously with the fermentation. Schwann (1837) described this organism independently, and Helmholtz (1843) confirmed the observation. They maintained that the process, in place of being a mere decomposition, was vital and depended on the presence of the organism they had discovered. This revolutionary doctrine was further elaborated pre-eminently by Pasteur and by Schultz, Schroeder, Dusch, Lister, Tyndall, and others. Their researches showed that fermentation was caused by the presence of these organisms; that the exclusion of these from fluids capable of fermentation, by various methods of sterilisation and filtration of the air in which they were abundantly present, was sufficient to prevent its occurrence; that the doctrine which attributed the production of fermentation to the influence of certain gases—e.g. oxygen (Gay-Lussac)—was erroneous; that the idea of the spontaneous generation (see SPONTANEOUS GENERATION) of such organisms within properly sterilised and protected fluids (Needham, Bastian, Pouchet, Huizinga) was fallacious; and that the so-called putrefaction was but one variety of fermentation.
(2) One result of these discussions was to develop a refinement of the methods of microscopical research, more especially with reference to the investigation of the lowest forms of life (see BACTERIA). Though bacteria had been recognised and described in the 17th century (Leeuwenhoek), it is mainly to the researches of the latter half of the 19th century that we are indebted for an approach to an accurate knowledge of the life-history of these organisms. By the masterly labours of Cohn, De Bary, Zopf, Van Tieghem, Nägeli, Klebs, Koch, and many others, the methods of demonstration have been improved to an extraordinary degree. The elaboration of staining methods alone, in conjunction with the use of perfected lenses, has made possible the detection and examination of minute organisms hitherto unrecognisable.
It is impossible to say when the idea of an analogy between the familiar phenomena of fermentation and those of acute disease first arose. It is certain that before the 19th century there had been prevalent an ill-defined feeling after something of the kind. More than two hundred years ago Robert Boyle (1627–91), in his 'Essay on the Pathological Part of Physik,' clothes the idea in words which, as Tyndall has said, 'have in them the foreast of prophecy.' The idea received more definite formulation in consequence of the researches into the nature of fermentation just referred to. In 1848 Fuchs stated that he had discovered bacteria in animals which had died of septicæmia. In 1850 it was announced (Davaine, Brannell, Pollender) that bacilli had been detected in the carcasses of animals affected with anthrax. The discovery was corroborated by various observers. But it was not till the disease had been induced by the inoculation of healthy animals with a minimal quantity of the organism (Davaine) that the Bacillus anthracis was recognised as the cause of the disease. Thus was afforded the first substantial proof of the germ theory. This success inspired further research on kindred lines. In comparatively quick succession other discoveries were announced, till, in 1882, Koch described the Bacillus tuberculosis as the organism responsible for the scourge of consumption, and in 1883 the bacillus of cholera.
Emphasis must be laid on the statement that the discovery of an organism in the circulation or tissues of a diseased animal cannot be accepted as proving the causal efficacy of the former. Apart from further experiment, it were perfectly fair to argue that such organism was a mere accompaniment of the morbid state, flourishing on the dying or diseased tissues. And, in fact, such secondary factors are recognised. It has, moreover, frequently happened that competing claims have been advanced in explanation of the same disease. It was necessary, therefore, that there should be formulated (Klebs, Koch) certain conditions, since known as Koch's postulates, which must be fulfilled by an organism whose causal relationship with a given disease is maintained. These are as follows: (1) The organism must be demonstrated in the circulation or tissues of the diseased animal; (2) the organism, so demonstrated, must be capable of artificial cultivation in suitable media outside the body, and successive generations of pure cultivation obtained; (3) such pure cultivation must, when introduced into a healthy and susceptible animal, produce the given disease; (4) the organism must again be found in the circulation or tissues of the inoculated animal. The claims of organisms which fail to meet these demands must be set aside to await further proof.
The number of diseases whose specific origin is now generally admitted is comparatively large, but of few of these can we speak with the same certainty as may be done regarding consumption (tuberculosis) and splenic fever (anthrax). In other words, the fulfilment of all four postulates by many of them has not been demonstrated or has been disputed. Besides anthrax and tuberculosis, the list includes leprosy, cholera (Asiatic), relapsing fever, typhoid fever, yellow fever, malaria, diphtheria, dysentery, syphilis, acute pneumonia, gonorrhea, septicæmia, erysipelas, actinomycosis, &c. With considerable probability we may add whooping-cough, measles, scarlatina, typhus, smallpox, hydrophobia, tetanus, British cholera, &c.; but the evidence regarding these and others is defective, and, in some cases, less substantive than analogical.
The specific organisms associated more or less exactly with those diseases are members of the groups (a) Coccaceæ and (b) Bacteriaceæ (see BACTERIA).
The admission that certain diseases are due to the presence and action of specific living organisms raises the further questions: (1) How do they enter the body? (2) How do they act?
(1) How do they enter the body? It has been conclusively shown that the Bacillus tuberculosis may obtain access by the inhalation of germ-laden air, by the ingestion of affected milk and possibly of tubercular meat, perhaps, too, through a cut or sore. It seems also likely that the bacilli may be transmitted from mother to fetus by way of the circulation. Similar lines of attack may be predicated of all the pathogenic organisms. Notably, in connection with wounds, it is important to bear in mind the possibility of infection with the germs which induce septicæmia—a fact on which was based the great advance in surgery associated with the name of Lister. See ANTI-SEPTIC SURGERY.
The possibility of infection varies much according to the conditions of growth of the particular organism and the receptivity of the host. This explains, on the one hand, the popularly accepted view that certain diseases are much more infective than others. Thus, typhoid fever differs widely from scarlatina in respect of degree of contagiousness. On the other hand, some persons undoubtedly are more susceptible to the attacks of certain organisms. Thus, among the subjects of tuberculosis, it is probable that preparedness of soil plays an important part in the production of the disease. And so with other pathogenic organisms. These processes have their analogy in the more common phenomena of vegetable life. Sow some seeds and they will germinate and grow on any soil, however unlikely. Other seeds may be scattered profusely, but will not develop, unless the soil has been carefully prepared and the other conditions of growth be fulfilled. It is impossible to enter here on the discussion of those conditions. Necessarily they vary much with different organisms. But it is important to realise the extreme value, from the therapeutic point of view, of their careful study. The first step to a rational treatment of such diseases is to know the responsible organism. This knowledge must include not only its shape and other physical characters, but the life-history of the microbe, and the conditions which assist or retard its development and reproduction. Such knowledge affords the only sound basis for a system of preventive medicine, which constitutes one of the most important departments of practical hygiene. Although still in its infancy, the preventive treatment of endemic, epidemic, and other contagious diseases has now become scientific.
(2) How do the organisms act? This is a much-debated question. It has been the subject of some of the most valuable of recent researches in this department. Do they act mechanically as irritants? Or is their action privative, by stealing from the tissues elements which are necessary to their development? Or have they a power of elaborating (or secreting) new products, which exert a toxic influence on the affected body? This last view is supported by weighty evidence and by the analogy of the fermentation processes already referred to. It would therefore seem that the microbe has the power of disturbing—or rather that, in order to the preservation of its own life, the microbe is compelled to disturb—the molecular arrangement of the elements in the medium in which it is developing. The products thus elaborated have been termed Ptomaines (Ptōma). They were so named by Selmi, who discovered their presence in the dead body during various stages of putrefaction. The ptomaine doctrine has been accepted in explanation of the process of septicæmia, and there is good reason for extending its application to the other infective processes. It is essential, however, to remember that, after the microbe has succeeded in invading the tissues, its further progress is not unopposed. There is a constant warfare between the living cells of the host and the living and multiplying cells of the invader, the contest being decided in favour of the stronger. The researches of Metschnikoff and others seem to show that the bacilli can be destroyed by the white corpuscles of the blood.
Granted that the organisms have entered the tissues or circulation, there still remain for the physician two modes of attack: (a) by attempting to exterminate the microbe itself through such agents as may be discovered to be possessed of germicidal properties; (b) by endeavouring to antagonise the poison which the microbe is distributing through the system. Many difficulties attend both methods, inasmuch as agents sufficiently potent to effect either object are themselves likely to prove injurious to the infected tissues. The aim of curative medicine is the discovery of remedies capable of preventing the growth of the microbe, yet innocuous to the host.
Reference must be made, in conclusion, to the question of immunity. It is well ascertained that certain animals are not susceptible to the attacks of certain pathogenic organisms, and that others suffer comparatively slightly. In man there may be traced the occurrence of individual immunity. Such facts have not yet received a satisfactory explanation. The almost universal immunity after a first attack of certain fevers and the comparative immunity from smallpox conferred by Vaccination (q.v.) are of interest in this connection. The experiments of Pasteur and others on Bacillus anthracis indicate that by repeated cultivation under special conditions it is possible to lessen the virulence of the most virulent of organisms and that inoculation with this altered bacillus confers immunity against further attack. More striking still are the experiments of Pasteur in connection with rabies (Hydrophobia, q.v.). By a special method that observer has accomplished an attenuation of the virus—the microbe not having been determined—whereby the worst features of the disease are disturbed. By this means it has been found possible in cases of infection to anticipate a serious attack by the introduction of this modified virus. In explanation of this it has been supposed that a poisonous ptomaine is germinated during the process, which, when injected in quantity during the stage of incubation of the disease, prevents the development of the suppurative germ. Those and other kindred observations disclose a most hopeful development of the germ theory in the direction of preventive inoculation.
The literature is a very large one. For general purposes the following may be consulted: Tyndall, Essays on the Floating Matter of the Air; Watson Cheyne, Antiseptic Surgery; Pasteur, Studies on Fermentation; Duclaux, Ferments et Maladies; Flügge, Fermente und Mikroparasiten; Schützenberger, Les Fermentations; Gussenbauer, Pyo-hämie und Pyo-Sephtämie; and the works of Lister, Klein, &c.