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 flagellæ. The spirochæte 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 aërobic bacilli or aërobes; those that cannot live in the presence of oxygen are spoken of as anaërobes. The great majority of bacteria, however, while they prefer to have oxygen, are able to live without it, and are called facultative anaërobes.
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.