Disease and Its Causes by William Thomas Councilman (early reader books .TXT) 📕

surfaces flattened, which cause gonorrhoea. e, f, g, h, Rod-shaped bacteria or bacilli: (e) diphtheria bacilli; (f) tubercle bacilli; (g) anthrax bacilli; (h) the same bacilli in cultures and producing spores; a small group of spores is shown. (i) Cholera spirillæ. (j) Typhoid bacilli. (k) Tetanus bacillus; i, j, k are actively motile, motion being effected by the small attached threads. (l) The screw-shaped spirochite which is the cause of syphilis.

The bacteria (Fig. 17) are unicellular organisms and vary greatly in size, shape and capacity of growth. The smallest of the pathogenic or disease-producing bacteria is the influenza bacillus, 1/51000 of an inch in length and 1/102000 of an inch in thickness; and among the largest is a bacillus causing an animal disease which is 1/2000 of an inch in length and 1/25000 of an inch in diameter. Among the free-living non-pathogenic forms much larger examples are found. In shape bacteria are round, or rod-shaped, or spiral; the round forms are called micrococci, the rod-shaped bacilli and the spiral forms are called spirilli. A clearer idea of the size is possibly given by the calculation that a drop of water would contain one billion micrococci of the usual size. Their structure in a general way conforms with that of other cells. On the outside is a cell membrane which encloses cytoplasm and nucleus; the latter, however, is not in a single mass, but the nuclear material is distributed through the cell. Many of the bacteria have the power of motion, this being effected by small hair-like appendages or flagellæ which may be numerous, projecting from all parts of the organisms or from one or both ends, the movement being produced by rapid lashing of these hairs. A bacterium grows until it attains the size of the species, when it divides by simple cleavage at right angles to the long axis forming two individuals. In some of the spherical forms division takes place alternately in two planes, and not infrequently the single individuals adhere, forming figures of long threads or chains or double forms. The rate of growth varies with the species and with the environment, and under the best conditions may be very rapid. A generation, that is, the interval between divisions, has been seen to take place in twenty minutes. At this rate of growth from a single cholera bacillus sixteen quadrillion might arise in a single day. Such a rate of growth is extremely improbable under either natural or artificial conditions, both from lack of food and from the accumulation in the fluid of waste products which check growth. Many species of bacteria in addition to this simple mode of multiplication form spores which are in a way analogous to the seeds of higher plants and are much more resistant than the simple or vegetative forms; they endure boiling water and even higher degrees of dry heat for a considerable time before they are destroyed. When these spores are placed in conditions favorable for bacterial life, the bacterial cells grow out from them and the usual mode of multiplication continues. This capacity for spore formation is of great importance, and until it was discovered by Cohn in 1876, many of the conditions of disease and putrefaction could not be explained. Spores, as the seeds of plants, often seem to be produced when the conditions are unfavorable; the bacterium then changes into this form, which under natural conditions is almost indestructible and awaits better days.

The bacteria are divided into species, the classification being based on their forms, on the mode of growth, the various substances which they produce and their capacity for producing disease. The differentiation of species in bacteria is based chiefly upon their properties, there being too little difference in form and size to distinguish species. The introduction of methods of culture was followed by an immediate advance of our knowledge concerning them. This method consists in the use of fluid and solid substances which contain the necessary salts and other ingredients for their food, and in or on which they are planted. The use of a solid or gelatinous medium for growth has greatly facilitated the separation of single species from a mixture of bacteria; a culture fluid containing sufficient gelatine to render it solid when cooled is sown with the bacteria to be tested by placing in it while warm and fluid, a small portion of material containing the bacteria, and after being thoroughly mixed the fluid is poured on a glass plate and allowed to cool. The bacteria are in this way separated, and each by its growth forms a single colony which can be further tested. It is self-evident that all culture material must be sterilized by heat before using, and in the manipulations care must be exercised to avoid contamination from the air. The refraction index of the bacterial cell is so slight that the microscopic study is facilitated or made possible by staining them with various aniline dyes. Owing to differences in the cell material the different species of bacteria show differences in the facility with which they take the color and the tenacity with which they retain it, and this also forms a means of species differentiation. The interrelation of science is well shown in this, for it was the discovery of the aniline dyes in the latter half of the nineteenth century which made the fruitful study of bacteria possible.

From the simplicity of structure it is not improbable that the bacteria are among the oldest forms of life, and all life has become adapted to their presence. They are of universal distribution; they play such an important part in the inter-relations of living things that it is probable life could not continue without them, at least not in the present way. They form important food for other unicellular organisms which are important links in the chain; they are the agents of decomposition, by which the complex substances of living things are reduced to elementary substances and made available for use; without them plant life would be impossible, for it is by their instrumentality that material in the soil is so changed as to be available as plant food; by their action many of the important foods of man, often those especially delectable, are produced; they are constantly with us on all the surfaces of the body; masses live on the intestinal surfaces and the excrement is largely composed of bacteria. It has been said that life would be impossible without bacteria, for the accumulation of the carcasses of all animals which have died would so encumber the earth as to prevent its use; but the folly of such speculation is shown by the fact that animals would not have been there without bacteria. It has been shown, however, that the presence of bacteria in the intestine of the higher animals is not essential for life. The coldest parts of the ocean are free from those forms which live in the intestines, and fish and birds inhabiting these regions have been found free from bacteria; it has also been found possible to remove small animals from their mother by Cæsarian section and to rear them for a few weeks on sterilized food, showing that digestion and nutrition may go on without bacteria.

Certain species of bacteria are aërobic, that is, they need free oxygen for their growth; others are anaërobic and will not grow in the presence of oxygen. Most of the bacteria which produce disease are facultative, that is, they grow either with or without oxygen; but certain of them, as the bacillus of tetanus, are anaërobic. There is, of course, abundance of oxygen in the blood and tissues, but it is so combined as to be unavailable for the bacteria. Bacteria may further be divided into those which are saprophytic or which find favorable conditions for life outside of the body, and the parasitic. Many are exclusively parasitic or saprophytic, and many are facultative, both conditions of living being possible. It has been found possible by varying in many ways the character of the culture medium and temperature to grow under artificial conditions outside of the body most, if not all, of the bacteria which cause disease. Thus, such bacteria as tubercle bacilli and the influenza bacillus can be cultivated, but they certainly would not find natural conditions which would make saprophytic growth possible.

Bacteria may be very sensitive to the presence of certain substances in the fluid in which they are growing. Growth may be inhibited by the smallest trace of some of the metallic salts, as corrosive sublimate, although the bacteria themselves are not destroyed. If small pieces of gold foil be placed on the surface of prepared jelly on which bacteria have been planted, no growth will take place in the vicinity of the gold foil.

Variations can easily be produced in bacteria, but they do not tend to become established. In certain of the bacterial species there are strains which represent slight variations from the type but which are not sufficient to constitute new species. If the environment in which bacteria are living be unusual and to a greater or less degree unfavorable, those individuals in the mass with the least power of adaptibility will perish, those more resistant and with greater adaptability will survive and propagate; and the peculiarity being transmitted a new strain will arise characterized by this adaptability. Bacteria with slight adaptability to the environment of the tissues and fluids of the animal body can, by repeated inoculations, become so adapted to the new environment as to be in a high degree pathogenic. In such a process the organisms with the least power of adaptation are destroyed and new generations are formed from those of greater power of adaptation. When bacteria are caused to grow in a new environment they may acquire new characteristics. The anthrax bacilli find the optimum conditions for growth at the temperature of the animal body, but they will grow at temperatures both above and below this. Pasteur found that by gradually increasing the temperature they could be grown at one hundred and ten degrees. When grown at this temperature they were no longer so virulent and produced in animals a mild non-fatal form of anthrax which protected the animal when inoculated with the virulent strain. The well known variations in the character of disease, shown in differences in severity and ease of transmission, seen in different years and in different epidemics, may be due to many conditions, but probably variation in the infecting organisms is the most important.

The protozoa, like the bacteria, are unicellular organisms and contain a nucleus as do all cells. They vary in size from forms seen with difficulty under the highest power of the microscope to forms readily seen with the unaided eye. Their structure in general is more complex than is the structure of bacteria, and many show extreme differentiation of parts of the single cells, as a firm exterior surface or cuticle, an internal skeleton, organs of locomotion, mouth and digestive organs and organs of excretion. They are more widely distributed than are the bacteria, and found from pole to pole in all oceans and in all fresh water. There are many modes of multiplication, and these are often extremely complicated. The most general mode and one which is common to all is by simple division; a modification of this is by budding in which projections or buds form on the body and after separation become new organisms. In other cases spores form within the cell which become free and develop further into complete organisms. These simple modes of multiplication often alternate in the same organism with sexual differentiation and conjugation. There is never a permanent sexual differentiation, but the sexual forms develop from a simple and non-sexual organism. Usually the sexual forms develop only in a special environment; thus the protozoon which in man is the cause of malaria, multiplies in the human blood by simple division, but in the body of