The influence of micro-organisms in the causation of disease, and the rōle played by them in interfering with the natural process of repair, are so important that the science of applied bacteriology has now come to dominate every department of surgery, and it is from the standpoint of bacteriology that nearly all surgical questions have to be considered.
The term sepsis as now used in clinical surgery no longer retains its original meaning as synonymous with “putrefaction,” but is employed to denote all conditions in which bacterial infection has taken place, and more particularly those in which pyogenic bacteria are present. In the same way the term aseptic conveys the idea of freedom from all forms of bacteria, putrefactive or otherwise; and the term antiseptic is used to denote a power of counteracting bacteria and their products.
General Characters of Bacteria.—A bacterium consists of a finely granular mass of protoplasm, enclosed in a thin gelatinous envelope. Many forms are motile—some in virtue of fine thread-like flagella, and others through contractility of the protoplasm. The great majority multiply by simple fission, each parent cell giving rise to two daughter cells, and this process goes on with extraordinary rapidity. Other varieties, particularly bacilli, are propagated by the formation of spores. A spore is a minute mass of protoplasm surrounded by a dense, tough membrane, developed in the interior of the parent cell. Spores are remarkable for their tenacity of life, and for the resistance they offer to the action of heat and chemical germicides.
Bacteria are most conveniently classified according to their shape. Thus we recognise (1) those that are globular—cocci; (2) those that resemble a rod—bacilli; (3) the spiral or wavy forms—spirilla.
Cocci or micrococci are minute round bodies, averaging about 1 µ in diameter. The great majority are non-motile. They multiply by fission; and when they divide in such a way that the resulting cells remain in pairs, are called diplococci, of which the bacteria of gonorrhœa and pneumonia are examples (Fig. 5). When they divide irregularly, and form grape-like bunches, they are known as staphylococci, and to this variety the commonest pyogenic or pus-forming organisms belong (Fig. 2). When division takes place only in one axis, so that long chains are formed, the term streptococcus is applied (Fig. 3). Streptococci are met with in erysipelas and various other inflammatory and suppurative processes of a spreading character.
Bacilli are rod-shaped bacteria, usually at least twice as long as they are broad (Fig. 4). Some multiply by fission, others by sporulation. Some forms are motile, others are non-motile. Tuberculosis, tetanus, anthrax, and many other surgical diseases are due to different forms of bacilli.
Spirilla are long, slender, thread-like cells, more or less spiral or wavy. Some move by a screw-like contraction of the protoplasm, some by flagellę. The spirochęte associated with syphilis (Fig. 36) is the most important member of this group.
Conditions of Bacterial Life.—Bacteria require for their growth and development a suitable food-supply in the form of proteins, carbohydrates, and salts of calcium and potassium which they break up into simpler elements. An alkaline medium favours bacterial growth; and moisture is a necessary condition; spores, however, can survive the want of water for much longer periods than fully developed bacteria. The necessity for oxygen varies in different species. Those that require oxygen are known as aėrobic bacilli or aėrobes; those that cannot live in the presence of oxygen are spoken of as anaėrobes. The great majority of bacteria, however, while they prefer to have oxygen, are able to live without it, and are called facultative anaėrobes.
The most suitable temperature for bacterial life is from 95° to 102° F., roughly that of the human body. Extreme or prolonged cold paralyses but does not kill micro-organisms. Few, however, survive being raised to a temperature of 134½° F. Boiling for ten to twenty minutes will kill all bacteria, and the great majority of spores. Steam applied in an autoclave under a pressure of two atmospheres destroys even the most resistant spores in a few minutes. Direct sunlight, electric light, or even diffuse daylight, is inimical to the growth of bacteria, as are also Röntgen rays and radium emanations.
Pathogenic Properties of Bacteria.—We are now only concerned with pathogenic bacteria—that is, bacteria capable of producing disease in the human subject. This capacity depends upon two sets of factors—(1) certain features peculiar to the invading bacteria, and (2) others peculiar to the host. Many bacteria have only the power of living upon dead matter, and are known as saphrophytes. Such as do nourish in living tissue are, by distinction, known as parasites. The power a given parasitic micro-organism has of multiplying in the body and giving rise to disease is spoken of as its virulence, and this varies not only with different species, but in the same species at different times and under varying circumstances. The actual number of organisms introduced is also an important factor in determining their pathogenic power. Healthy tissues can resist the invasion of a certain number of bacteria of a given species, but when that number is exceeded, the organisms get the upper hand and disease results. When the organisms gain access directly to the blood-stream, as a rule they produce their effects more certainly and with greater intensity than when they are introduced into the tissues.
Further, the virulence of an organism is modified by the condition of the patient into whose tissues it is introduced. So long as a person is in good health, the tissues are able to resist the attacks of moderate numbers of most bacteria. Any lowering of the vitality of the individual, however, either locally or generally, at once renders him more susceptible to infection. Thus bruised or torn tissue is much more liable to infection with pus-producing organisms than tissues clean-cut with a knife; also, after certain diseases, the liability to infection by the organisms of diphtheria, pneumonia, or erysipelas is much increased. Even such slight depression of vitality as results from bodily fatigue, or exposure to cold and damp, may be sufficient to turn the scale in the battle between the tissues and the bacteria. Age is an important factor in regard to the action of certain bacteria. Young subjects are attacked by diphtheria, tuberculosis, acute osteomyelitis, and some other diseases with greater frequency and severity than those of more advanced years.
In different races, localities, environment, and seasons, the pathogenic powers of certain organisms, such as those of erysipelas, diphtheria, and acute osteomyelitis, vary considerably.
There is evidence that a mixed infection—that is, the introduction of more than one species of organism, for example, the tubercle bacillus and a pyogenic staphylococcus—increases the severity of the resulting disease. If one of the varieties gain the ascendancy, the poisons produced by the others so devitalise the tissue cells, and diminish their power of resistance, that the virulence of the most active organisms is increased. On the other hand, there is reason to believe that the products of certain organisms antagonise one another—for example, an attack of erysipelas may effect the cure of a patch of tuberculous lupus.
Lastly, in patients suffering from chronic wasting diseases, bacteria may invade the internal organs by the blood-stream in enormous numbers and with great rapidity, during the period of extreme debility which shortly precedes death. The discovery of such collections of organisms on post-mortem examination may lead to erroneous conclusions being drawn as to the cause of death.
Results of Bacterial Growth.—Some organisms, such as those of tetanus and erysipelas, and certain of the pyogenic bacteria, show little tendency to pass far beyond the point at which they gain an entrance to the body. Others, on the contrary—for example, the tubercle bacillus and the organism of acute osteomyelitis—although frequently remaining localised at the seat of inoculation, tend to pass to distant parts, lodging in the capillaries of joints, bones, kidney, or lungs, and there producing their deleterious effects.
In the human subject, multiplication in the blood-stream does not occur to any great extent. In some general acute pyogenic infections, such as osteomyelitis, cellulitis, etc., pure cultures of staphylococci or of streptococci may be obtained from the blood. In pneumococcal and typhoid infections, also, the organisms may be found in the blood.
It is by the vital changes they bring about in the parts where they settle that micro-organisms disturb the health of the patient. In deriving nourishment from the complex organic compounds in which they nourish, the organisms evolve, probably by means of a ferment, certain chemical products of unknown composition, but probably colloidal in nature, and known as toxins. When these poisons are absorbed into the general circulation they give rise to certain groups of symptoms—such as rise of temperature, associated circulatory and respiratory derangements, interference with the gastro-intestinal functions and also with those of the nervous system—which go to make up the condition known as blood-poisoning, toxęmia, or bacterial intoxication. In addition to this, certain bacteria produce toxins that give rise to definite and distinct groups of symptoms—such as the convulsions of tetanus, or the paralyses that follow diphtheria.
Death of Bacteria.—Under certain circumstances, it would appear that the accumulation of the toxic products of bacterial action tends to interfere with the continued life and growth of the organisms themselves, and in this way the natural cure of certain diseases is brought about. Outside the body, bacteria may be killed by starvation, by want of moisture, by being subjected to high temperature, or by the action of certain chemical agents of which carbolic acid, the perchloride and biniodide of mercury, and various chlorine preparations are the most powerful.
Immunity.—Some persons are insusceptible to infection by certain diseases, from which they are said to enjoy a natural immunity. In many acute diseases one attack protects the patient, for a time at least, from a second attack—acquired immunity.
Phagocytosis.—In the production of immunity the leucocytes and certain other cells play an important part in virtue of the power they possess of ingesting bacteria and of destroying them by a process of intra-cellular digestion. To this process Metchnikoff gave the name of phagocytosis, and he recognised two forms of phagocytes: (1) the microphages, which are the polymorpho-nuclear leucocytes of the blood; and (2) the macrophages, which include the larger hyaline leucocytes, endothelial cells, and connective-tissue corpuscles.
During the process of phagocytosis, the polymorpho-nuclear leucocytes in the circulating blood increase greatly in numbers (leucocytosis), as well as in their phagocytic action, and in the course of destroying the bacteria they produce certain ferments which enter the blood serum. These are known as opsonins or alexins, and they act on the bacteria by a process comparable to narcotisation, and render them an easy prey for the phagocytes.
Artificial or Passive Immunity.—A form of immunity can be induced by the introduction of protective substances obtained from an animal which has been actively immunised. The process by which passive immunity is acquired depends upon the fact that as a result of the reaction between the specific virus of a particular disease (the antigen) and the tissues of the animal attacked, certain substances—antibodies—are produced, which when transferred to the body of a susceptible animal protect it against that disease. The most important of these antibodies are the antitoxins. From the study of the processes by which immunity is secured against the effects of bacterial action the serum and vaccine methods of treating certain infective diseases have been evolved. The serum treatment is designed to furnish the patient with a sufficiency of antibodies to neutralise the infection. The anti-diphtheritic and the anti-tetanic act by neutralising the specific toxins of the disease—antitoxic serums; the anti-streptcoccic and the serum for anthrax act upon the bacteria—anti-bacterial serums.
A polyvalent serum, that is, one derived from an animal which has been immunised by numerous strains of the organism derived from various sources, is much more efficacious than when a single strain has been used.
Clinical Use of Serums.—Every precaution must be taken to prevent organismal contamination of the serum or of the apparatus by means of which it is injected. Syringes are so made that they can be sterilised by boiling. The best situations for injection are under the skin of the abdomen, the thorax, or the buttock, and the skin should be purified at the seat of puncture. If the bulk of the full dose is large, it should be divided and injected into different parts of the body, not more than 20 c.c. being injected at one place. The serum may be introduced directly into a vein, or into the spinal canal, e.g. anti-tetanic serum. The immunity produced by injections of antitoxic sera lasts only for a comparatively short time, seldom longer than a few weeks.
“Serum Disease” and Anaphylaxis.—It is to be borne in mind that some patients exhibit a supersensitiveness with regard to protective sera, an injection being followed in a few days by the appearance of an urticarial or erythematous rash, pain and swelling of the joints, and a variable degree of fever. These symptoms, to which the name serum disease is applied, usually disappear in the course of a few days.
The term anaphylaxis is applied to an allied condition of supersensitiveness which appears to be induced by the injection of certain substances, including toxins and sera, that are capable of acting as antigens. When a second injection is given after an interval of some days, if anaphylaxis has been established by the first dose, the patient suddenly manifests toxic symptoms of the nature of profound shock which may even prove fatal. The conditions which render a person liable to develop anaphylaxis and the mechanism by which it is established are as yet imperfectly understood.
Vaccine Treatment.—The vaccine treatment elaborated by A. E. Wright consists in injecting, while the disease is still active, specially prepared dead cultures of the causative organisms, and is based on the fact that these “vaccines” render the bacteria in the tissues less able to resist the attacks of the phagocytes. The method is most successful when the vaccine is prepared from organisms isolated from the patient himself, autogenous vaccine, but when this is impracticable, or takes a considerable time, laboratory-prepared polyvalent stock vaccines may be used.
Clinical Use of Vaccines.—Vaccines should not be given while a patient is in a negative phase, as a certain amount of the opsonin in the blood is used up in neutralising the substances injected, and this may reduce the opsonic index to such an extent that the vaccines themselves become dangerous. As a rule, the propriety of using a vaccine can be determined from the general condition of the patient. The initial dose should always be a small one, particularly if the disease is acute, and the subsequent dosage will be regulated by the effect produced. If marked constitutional disturbance with rise of temperature follows the use of a vaccine, it indicates a negative phase, and calls for a diminution in the next dose. If, on the other hand, the local as well as the general condition of the patient improves after the injection, it indicates a positive phase, and the original dose may be repeated or even increased. Vaccines are best introduced subcutaneously, a part being selected which is not liable to pressure, as there is sometimes considerable local reaction. Repeated doses may be necessary at intervals of a few days.
The vaccine treatment has been successfully employed in various tuberculous lesions, in pyogenic infections such as acne, boils, sycosis, streptococcal, pneumococcal, and gonococcal conditions, in infections of the accessory air sinuses, and in other diseases caused by bacteria.