Chapter VI
Classification Of The Organisms Which Cause Disease.—Bacteria: Size, Shape, Structure, Capacity For Growth, Multiplication And Spore Formation.—The Artificial Cultivation Of Bacteria.—The Importance Of Bacteria In Nature.—Variations In Bacteria.—Saprophytic And Parasitic Forms.—Protozoa.—Structure More Complicated Than That Of Bacteria.—Distribution In Nature.—Growth And Multiplication.— Conjugation And Sexual Reproduction.—Spore Formation.—The Necessity For A Fluid Environment.—The Food Of Protozoa.—Parasitism.—The Ultra-Microscopic Or Filterable—Organisms.—The Limitation Of The Microscope.—Porcelain Filters To Separate Organisms From A Fluid.— Foot And Mouth Disease Produced By An Ultra-Microscopic Organism.— Other Diseases So Produced.—Do New Diseases Appear?
The living organisms which cause the infectious diseases are classified under bacteria, protozoa, yeasts, moulds, and ultra-microscopic organisms. It is necessary to place in a separate class the organisms whose existence is known, but which are not visible under the highest powers of the microscope, and have not been classified. The yeasts and moulds play a minor part in the production of disease and cannot be considered in the necessary limitation of space.
Fig. 17.—Various Forms Of Bacteria. a, b, c, d, Round bacteria or cocci: (a) Staphylococci, organisms which occur in groups and a common cause of boils; (b) streptococci, organisms which occur in chains and produce erysipelas and more severe forms of inflammation; (c) diplococci, or paired organisms with a capsule, which cause acute pneumonia; (d) gonococci, with the opposed 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 the mosquito multiplication by sexual differentiation takes place. Under no conditions is multiplication so rapid as with the bacteria, and in general the simpler the form of organism the more rapid is the multiplication. It is common to all of the protozoa to develop forms which have great powers of resistance, this being due in some cases to encystment, in which condition a resistant membrane is formed on the outside, in others to the production of spores. A fluid environment is essential to the life of the protozoa, but the resistant forms can endure long periods of dryness or other unfavorable environmental conditions. The universal distribution of the protozoa is due to this; the spores or cysts can be carried long distances by the wind and develop into active forms when they reach an environment which is favorable. Their distribution in water depends upon the amount of organic material this contains. In pure drinking water there may be very few, but in stagnant water they are very numerous, living not on the organic material in solution in this, but on the bacteria which find in such fluid favorable conditions for existence. The food of protozoa consists chiefly of other organisms, particularly bacteria, and they are classed with the animals. The protozoa are the most widely distributed and the most universal of the parasites. The infectious diseases which they produce in man, although among the most serious are less in number than those produced by bacteria. So marked is the tendency to parasitism that they are often parasitic for each other, smaller forms entering into and living upon the larger. Variation does not seem to be so marked in the protozoa as in the bacteria, though this is possibly due to our greater ignorance of them as a class. We are not able, except in rare instances, to grow them in pure culture, and study innumerable generations under changes in the environment, as the bacteria have been studied.
If we regard the living things on earth from the narrow point of view as to whether they are necessary or useless or hostile to man, the protozoa must be regarded as about the least useful members of the biological society. It is very possible that such a conclusion is due to ignorance; so closely are all living things united, so dependent is one form of cell activity upon other forms that it is impossible to foretell the result of the removal of a link. The protozoa do not seem to be as necessary for the life of man as are the bacteria; they produce many of the diseases of man, many of the diseases of animals on which man depends for food; they cause great destruction in plant life, and in the soil they feed upon the useful bacteria. It is well to remember, however, that fifty years ago several of the organs of the body whose activity we now recognize as furnishing substances necessary for life were regarded as useless members and, since they became the seat of tumors, as dangerous members of the body. The only organ which now seems to come into such a class is the vermiform appendix, and its lowly position among organs is due merely to an unhappy accident of development.
The class of organisms known as the filterable viruses or the ultra-microscopic or the invisible organisms have a special interest in many ways. The limitation in the power of the microscope for the study of minute objects is due not to a defect in the instrument but to the length of the wave of light. It is impossible to see clearly under the microscope using white light, objects which are smaller in diameter than the length of the wave which gives a limit of 0.5µ. or 1/125,000 of an inch. By using waves of shorter length, as the ultra-violet light, objects of 0.1µ. or 1/250000 of an inch can be seen; but as these methods depend upon photography for the demonstration of the object the study is difficult. The presence of objects still smaller than 0.1 m. can be detected in a fluid by the use of the dark field illumination and the ultra-microscope, the principle of which is the direction of a powerful oblique ray of light into the field of the microscope. The objects are not visible as such, but the dispersion of the light by their presence is seen.
The demonstration that infectious diseases were produced by organisms so small as to be beyond demonstration with the best microscopes was made possible by showing, that some fluid from a diseased animal was infectious; and capable of producing the disease when inoculated into a susceptible animal. The fluid was then filtered through porcelain filters which were known to hold back all objects of the size of the smallest bacteria and the disease produced by inoculating with the clear filtrate. There are a number of such filters of different degrees of porosity manufactured, and they are often used to procure pure water for drinking, for which use they are more or less, generally however, less efficacious. The filter has the form of a hollow cylinder and the liquid to be filtered is forced through it under pressure. For domestic use the filter is attached by its open end to the water tap and the pressure from the mains forces the water through it. In laboratory uses, denser filters of smaller diameters are used, and the filter is surrounded by the fluid to be tested. The open end of the filter passes into a vessel from which the air is exhausted and filtration takes place from without inward. The test of the effectiveness of the filter is made by adding to the filtering fluid some very minute and easily recognizable bacteria and testing the filtrate for their presence. These filters have been studied microscopically by grinding very thin sections and measuring the diameter of the spaces in the material. These are very numerous, and from 1/25000 to 1/1000 of an inch in diameter, spaces which would allow bacteria to pass through, but they are held back by the very fine openings between the spaces and by the tortuosity of the intercommunications. When the coarser of such filters have been long in domestic service in filtering drinking water, bacteria may grow in and through them giving greater bacterial content to the supposed bacteria-free filtrate than in the filtering water.
That an animal disease was due to such a minute and filterable organism was first shown by Loeffler in 1898 for the foot and mouth disease of cattle. This is one of the most infectious and easily communicable diseases. The lesions of the disease take the form of blisters which form on the lips and feet and in the mouths of cattle, and inoculation with minute quantities of the fluid in the blisters produces the disease. Loeffler filtered the fluid through porcelain filters, hoping to obtain a material which inoculated into other cattle would render them immune, and to his surprise found that the typical disease was produced by inoculating with the filtrate. Naturally the first idea was that the disease was caused by some soluble poison and not by a living organism, but this was disproved in a number of ways. The most powerful poison known is obtained from cultures of the tetanus bacillus of which 0.000,000,1 of a gram (one gram is 15.43 grains) kills a mouse, or one gram kills ten million mice. Loeffler found that 1/30 gram of the contents of the vesicles killed a calf of two hundred kilograms weight, and assuming that the essential poison was present in the fluid in one part to five hundred it would be several hundred times more powerful than the tetanus poison. Further, the disease produced by inoculation of the filtrate was itself inoculable and could be transmitted from animal to animal. It was also found that when the virus was filtered several times it ceased to be inoculable, showing that each time the fluid was passed through the filter some of the minute organisms contained in it were held back.
It is not known whether these organisms belong to the bacteria or protozoa, and naturally nothing is known as to their form, size and structure. Up to the present about twenty diseases are known to be due to a filterable virus, and among these are some of the most important for animals and for man. Among the human diseases, yellow fever, poliomyelitis, and dengue are so produced; of the animal diseases in addition to foot and mouth disease, pleuropneumonia, cattle plague, African horse sickness, several diseases of fowls and the mosaic disease of the tobacco plant have all been shown to be due to a filterable virus. Of these organisms the largest is that which produces pleuropneumonia in cattle, and this alone has been cultivated. It gives a slight opacity to the culture fluids, and when magnified two thousand diameters appears as a minute spiral or round or stellate organism having a variety of forms. Its size is such that it passes the coarse, but is held back by the finer, filters and it is possible that this does not belong to the same class with the others.[8] The diseases produced by the filterable viruses taken as a class show much similarity. They run an acute course, are severe, and the immunity produced by the attack endures for a long time.
Considered in its biological relations, infection is the adaptation of an organism to the environment which the body of the host offers. It is rather singular that variations in organisms represented by such adaptation do not more frequently arise, in which case new diseases would frequently occur. It cannot be denied that new diseases appear, but there is no certain evidence that they do, and there is equally no evidence that diseases disappear. From the meagre descriptions of diseases, usually of the epidemic type, which have come down to us from the past, it is difficult to recognize many of the diseases described. The single diseases are recognized by comparing the causes, the lesions and the symptoms with those of other diseases, and new diseases are constantly being separated off from other diseases having more or less common features. Many new diseases have been recognized and named, but it is always more than probable that previously they were confounded with other diseases. Smallpox is such a characteristic disease that one would think it would have been recognized as an entity from the beginning, but although the description of some of the epidemics in remote times conform more or less to the disease as we know it, the first accurate description is in the eighth century by the Arabian physician Rhazes. Cerebro-spinal meningitis was not recognized as a separate disease until 1803, diphtheria not until 1826, and the separation between typhoid and typhus fever was not made before 1840. Nor is it sure that any diseases have disappeared, although there seems to have been a change in the character of many. It is difficult to reconcile leprosy as it appears now with the universal horror felt towards it, due to the persistence of the old traditions. It is possible, however, that the disease has not changed its character, but that such diseases as smallpox, syphilis, and certain forms of tuberculosis were formerly confounded with leprosy, thus giving a false idea of its prevalence.
In certain cases the adaptation of the organism is for a narrow environment; for example, the parasitism may extend to a simple species only, in others the adaptation may extend to a number of genera. In certain cases the adaptation is mutual, extending to both parasite and host and resulting in symbiosis, and this condition may be advantageous for both. Certain of the protozoa harbor within them cells of algæ utilizing to their own advantage the green chlorophil of the algæ in obtaining energy from sunlight and in turn giving sustenance to the algæ. Although the algæ are useful guests, when they become too numerous the protozoan devours them. It is evident that symbiosis is the most favorable condition for the existence of the parasite, and an injurious action exerted by the parasite on the host unfavorable. The death of the host is an unfortunate incident from the parasite's point of view in that it is deprived of habitation and food supply, being placed in the same unfortunate situation as may befall a social parasite by the death of his host.