A microorganism or microbe, some species of which cause all disease, is a minute plant or animal too small, as a rule, to be visible to the naked eye.

The word germ may be used to designate any microorganism, but it has so many other meanings and has been so loosely employed even in this sense, that it cannot be used for accurate scientific description.

Bacteria are minute plants on the order of fungae, many of which are able to produce fermentation, decomposition or disease.

Although the word bacterium by derivation has the same meaning as bacillus and indicates a rod shaped fungus, it has been so loosely employed that it may very well be applied to the entire germal family, retaining the word bacillus in the narrower sense.

Description of Bacteria. Schizomycetes is the name given all the bacteria of putrefaction and disease, the former being called saprophytic and the latter pathogenic.

Bacteria are minute fungi, each consisting of a single cell enclosed in a cell membrane of cellulose which can be demonstrated by iodine, the latter causing the protoplasm to retract from the cell wall. There is no nucleus or central core. Some of the bacteria are colorless, others pigmented, yellow, blue or red. The cells vary in shape and in size in different species as well as in their mode of growth, and are named in accordance with these peculiarities. The round or oval cells are called cocci; the rod-shaped organisms are termed bacilli. The cocci are called micrococci or macrococci according to their size; diplococci or tetracocci, according to the production of pairs or groups of four in their multiplication; streptococci, because in their growth they always form chains of cells; staphylococci, because they grow in irregular clusters resembling bunches of grapes. Some of the bacteria have the power of motion generally produced by cilia or flagella and others are motionless.

Habitat. These organisms may truly be said to be omnipresent. Every thing we wear or use or eat, even the air itself, is impregnated with them. Pathogenic germs may also be found among these myriads. Every species has its own particular habitat, where the conditions especially favor its growth, just as any of the larger plants require a certain soil, a supply of water, temperature, and proper amount of light in order to make growth and multiplication possible.

The bacteria in the air are more numerous in dry weather, being carried up as dust by the wind, for a moist surface holds any bacteria which may lie upon it. So complete is the action of moisture, that air, which contained 600 microorganisms when inspired, has been shown to return from the lungs with almost none, the moist respiratory surfaces catching and holding the bacteria; so that the expired air is practically sterilized; this is true even when the expiration is from diseased lungs. The act of coughing, however, may expel bacteria in the mucus ejected. The number of bacteria in the air is very variable, but is much greater in houses than out of doors, and is naturally increased by attempts to clean the rooms.

Parasitic Nature. The number of species of pathogenic germs is comparatively small compared with the number of all the varieties of germs, for the latter are practically innumerable. Indeed, the wonderful qualities of resistance in animal tissues is the only thing that makes animal life possible and it is this power of resistance that allows certain wounds to heal by primary union when left without protection or care.

The schizomycetes are unable to extract nitrogen from the air or the soil, like the higher vegetables, and must, therefore, be provided with a higher nitrogenous compound, such as is produced by vegetable and animal life. Some of them are able to live upon dead organic matter, while others cannot exist without living tissues to feed upon and are therefore true parasites. There are some which are able to live upon either dead or living tissues and are known as facultative parasites, a class which includes a majority of pathogenic germs. Some organisms require albuminous matter, others need carbohydrates; they all require water, carbon, nitrogen, oxygen, and certain inorganic materials, especially lime and potassium. All organisms require water. If dried, no form will multiply, and many forms will die.

The fluids and tissues of the individual may or may not afford a favorable soil for the germs of a disease, or, in the same person afford it at one time, and not at another. Some individuals seem to possess indestructible immunity from, and others are especially prone to, certain contagious diseases. Impairment of health, by alterating some subtle condition of the soil, may make a person liable who previously was exempt.

Effect of Oxygen. Some bacteria need free oxygen; some can live either with or without free oxygen, while others cannot live at all in the presence of free oxygen. Those requiring oxygen are called aerobic; those which can live with or without it are called facultative aerobic; those which do not live in free oxygen are called anaerobic.

Bacteria are very sensitive to temperature, few being able to live in a temperature below 68°F. or 29°C. or above 104°F. or 40°C. The pathogenic varieties thrive best at about the normal temperature of the blood. Direct sunlight retards their growth and may kill them. Freezing renders bacteria motionless and incapable of multiplication, but it does not kill them; they again become active when the temperature is raised. The absurdity of employing cold as a germicide is evident when it is known that a temperature of 200°F. below zero is not fatal to germ life, cell activities by such a temperature only being rendered dormant. The high temperatures are fatal to bacteria, moist heat being more destructive than dry heat, and adult cells are more easily killed than spores. A temperature less than 212°F. will kill many organisms and boiling will kill every pathogenic organism that does not form spores. Some spores are not destroyed after prolonged boiling and some will withstand a temperature of 120°C. As a practical fact, however, boiling water kills in a few minutes all cocci, most bacilli, and all pathogenic spores, though anthrax and tetanus are harder to kill than are the spores of other bacteria.

Under favorable conditions bacteria multiply rapidly, but when conditions are unfavorable, they take on a spore formation and remain in a quiescent state, like the seed of a plant, waiting—it may be years—until proper conditions are present. The spores are protected by such a thick envelope and have such great potential vitality, that it is much more difficult to kill them than the developed bacteria. Certain spores that withstand 212°F. or 100°C., can be killed when fully developed at 130°F. or 55°C.

Toxins. As bacteria grow, certain poisonous chemical substances appear about them. These poisons are produced by them directly, or are formed in the organic matter or tissues in which they live, as the result of their presence. Some of these substances are alkaloidal and are known as ferments or ptomains. Others are albuminous in nature and are called toxalbumins. The ptomains and toxalbumins are exceedingly powerful poisons, producing local necrosis, inflammation and even suppuration, when introduced by themselves and entirely free from living germs, into the tissues of animals. Pathogenic bacteria abstract the lymph from the blood. As the lymph contains elements necessary to the body, such as water, oxygen, albumins, carbohydrates, etc., their loss brings about body-waste and exhaustion from lack of nourishment. Again, bacteria produce a vast number of compounds, some harmless and others highly poisonous.

The symptoms of a microbic disease are largely due to the absorption of poisonous materials from the area of infection. These poisons may be formed in the tissues by the action upon them of the bacteria, or they may be liberated from the bodies of degenerating microbes.

Bacteria secrete and contain ferments like pepsin or trypsin, and as albumoses are formed in the alimentary canal by the action of the digestive ferments upon proteids, sugars, and starches, so microbic albumoses are formed by the action of microbic ferments upon tissues.

The local and general symptoms of these toxins depend upon the particular toxin employed and a large number of these poisons have been isolated and studied. Those of the surgically important pathogenic germs, produce inflammation locally, with general symptoms of fever, chills, cardiac depression, irritation of the kidneys and bowels and cerebral symptoms, such as delirium and coma. The toxalbumins also appear to have the effect of destroying the bacteria to which they owe their origin when they have been produced in large quantity.

Cultivation. Bacteria are cultivated for study in the laboratory in meat extracts, in gelatine, or agar agar (a sort of vegetable gelatine), or raw potato, in blood serum and in other materials. The simplest method of cultivation is in bouillon, sterilized in flasks, with cotton plugs. Another method of studying bacteria is by the inoculation of animals.

Infection. Bacteria gain admission to the living tissues under natural conditions, by penetrating any of the mucous membranes which they can reach, or by entering open wounds. It may be said in general that an intact epidermis is almost a complete protection against infection, and that an intact mucous membrane is a good protection. This difference in vulnerability between the mucous membrane and the skin is important, and is probably due to the cornifaction of the epithelial cells, and to their numerous layers, as well as to the protection afforded by the thick corium. The single layer of soft mucous cells is much more easily penetrated.

Typhoid bacilli and other hostile germs have been actually observed in the urine, in the bile, in the intestinal secretions and in the saliva. The bacteria of typhoid fever and tuberculosis have been found in the milk of nursing mothers.

The local phenomena of inflammation usually follow the introduction of living bacteria into the tissues, and general symptoms of poisoning follow later, when the bacteria, toxins, or ptomains, have entered the circulation. Some bacteria, however, excite no local reaction, but enter the circulation at once. The pyogenic variety, it should be noted, cause the production of pus.

Elimination. Bacteria can be eliminated from the blood in several ways; the kidneys, however, are the organs which carry the burden of most frequently relieving the body of them. Even the sweat glands are supposed to eliminate both bacterial toxins and bacteria.

Resistance Offered by Tissues. The tissues have considerable power of resistance under ordinary circumstances, although the exact sources of this power are not well under stood. Phagocytosis—the power of destruction and removal of bacteria supposedly possessed by the leucocytes emigrating from the blood vessels—explains it in part. It is also accounted for by the germicidal properties of the blood serum.

The resistance of the tissues may in some cases be due to the absence from them of some particular element necessary to the growth of a particular microorganism. This refractoriness varies in every species of animal in its relation to every form of germ. Different individuals of one species also vary in their susceptibility, and even different parts of the body vary in the same individual. The lower animals offer a greater resistance to pyogenic bacteria than do human beings.

Any cause that lowers the vitality by depressing the system, reduces the resistance to bacteria and is therefore apt to favor their growth. Exhausting diseases such as anemia, obesity, alcoholism, diabetes, fatigue, or even exposure to cold, are instances. Germ growth is also favored by the presence of dead, or injured tissues, of blood clots, of foreign substances, and above all, by the presence of some of the substances in which the germ has already been growing at the time of its inoculation, and containing some of its toxins.

Immunity. To be able to resist the invasion of any species of bacteria, one is said to be refractory to or immune against that variety of germs.

Serum therapy is based on the demonstrated fact of immunity, and of the possibility of producing it by injecting the serum of immunized animals. In many infectious diseases, one attack protects an individual for a lifetime and one form of disease may protect against even a more virulent form, as vaccination protects against smallpox. It is a fact that if the serum of an animal which has been rendered immune to a certain disease be injected into a susceptible animal, the same immunity can be produced temporarily in the second animal. Serum therapy proves that the injected serum will not only confer immunity against the infection, but will enable the animal to throw off an already existing infection.

Sterilization. The question how to destroy microorganisms is one of the most important in bacteriology. Exactly how chemical antiseptics act in suspending the growth in living organisms and yet leaving them capable of restoration, is not understood. The explanation is offered that the antiseptics enter into combination with the capsule of the cell and can be freed from it by breaking up this chemical combination. It has always been evident that very minute quantities of germicidal substances, and some substances which are not germicidal, would prevent the growth of bacteria, so that it is not surprising that chemical disinfectants should act in this prolonged inhibitory way. It must be remembered that in operative surgical work, germs which will not develop are, for practical purposes, as good as dead; therefore such results do not invalidate the present methods of sterilization for operations. They naturally stimulate interest in the discovery of better methods of sterilization and especially in the thorough application of the methods upon which we are now depending, in order to obtain the best possible results from them. There are three ways of destroying microorganisms: (1) by deprivation of food and water, (2) by chemicals (including toxins), (3) by heat.

Chemical Antiseptics. For practical disinfection, chemicals and heat need only concern us. The power of these substances is greatly decreased by heat, grease, oil, mucus, and even blood will cover germs with a coating which prevents chemical germicides from reaching them. Among the ordinary germicides, bichloride of mercury, iodin, alcohol and carbolic acid, are of the greatest importance. A source of error in the direct application of these experiments is the fact that many of these chemicals are decomposed or rendered inert, by combinations with the albuminoids of blood and pus, mercuric bichloride being transformed into an indifferent substance and even carbolic acid being altered.

Carbolic Acid is a valuable germicide in the strength of from 1 to 40, to 1 to 20. It is very irritant to tissues and carbolized dressings may be responsible for the sloughing of a wound. It is inert in fatty tissues.

Carbolic acid is readily absorbed, and may thus produce toxic symptoms. One of the early signs of absorption is the appearance of the urine, which may assume a smoky, greenish or blackish hue. Examination shows a great diminution or entire absence of sulphates, when the acidulated urine is heated with chloride of barium. The urine also contains albumin. The appearance of the urine is an indication that the use of the drug must be discontinued.

Kreolin, a preparation made from coal tar, is a germicide without irritant or toxic effects. It is less powerful than carbolic acid, but acts similarly, and is used in emulsion of a strength of from 1 to 15%. It does not irritate the skin like carbolic acid.

Peroxide of Hydrogen is a most admirable agent for the destruction of pus cocci. It probably destroys the albuminous element upon which the bacteria live, and starves the fungi.

Peroxide is not fatal to tetanus bacilli.

Iodoform is largely used, but it is not a germicide as bacteria will grow upon it. It hinders the development of bacteria and directly antagonizes the toxic products of germ life.

Silver Nitrate is a valuable antiseptic. It exerts an inhibitive action upon the growth of microorganisms, but irritates the tissues.

Formaldehyde has valuable antiseptic properties. Formalin is a 40% solution of the gas in water. Solutions of this strength are very irritant to the tissues, but a 2% solution can be used to disinfect wounds and instruments.

Nucleins, especially protonuclein, possess germicidal powers. Protonuclein is of value in treating areas of infection, particularly when sloughing exists. A great many other antiseptics are used.

Heat. The surest and quickest method of destroying bacteria is by heat. Even the spores succumb to it. Anthrax spores are killed in 2 minutes in boiling water, and the various bacilli and cocci in from 2 to 5 seconds.

When a substance to be sterilized by heat will not bear so high a temperature, the method of fractional sterilization is employed, the fluid to be sterilized being heated to from 140°F. to 175°F. or to from 69°C. to 80°C., for from 15 to 30 minutes every 3 days or 7 days. The theory is that the adult germs are killed by the first heating and that any spores which develop subsequently are destroyed in their adult state at the next heating. The fluid, meanwhile, must be kept at an even temperature which will encourage the development of any spores it may contain. Even anthrax spores may be killed by 167°F. to 185°F., or 75°C. to 80°C., in a one and four-tenths solution of bicarbonate of soda, in from 8 to 20 minutes. Dry heat is not so efficient as moist heat.

The following are the more important bacteria we meet in surgical conditions:

The first six are known as pyogenic bacteria, as they all produce pus; in addition to the above there are many more microorganisms, but from a surgical standpoint those mentioned are the most important.

The staphylococcus pyogenes is a spherical coccus of somewhat variable size but averaging about 8 microns; when properly stained it can often be seen to be formed of two separate hemispheres. In pus it is generally found in small heaps containing from two to ten members, but it also occurs singly and in pairs, and even in short chains like the streptococcus, thus rendering diagnosis difficult with the microscope alone. Its cultures are of a yellowish tinge. The aureus type is the most usual cause of abscesses (circumscribed suppurations) and 77% of acute abscesses are due to the staphylococci.

The staphylococcus pyogenes aureus is a facultative anaerobic parasite which is widely distributed in nature, and is found in the soil, in the dust of air, in water, in the alimentary canal, under the nails, and in the superficial layers of the skin. It forms the characteristic color only when it grows in air. It is killed in ten minutes by a moist temperature of 58°C. and is instantly killed by boiling water. Carbolic acid (1 to 40) and bichloride of mercury (1 to 2000) are quickly fatal to these cocci.

Staphylococcus pyogenes citreus, the lemon-colored coccus, is found occasionally in acute circumscribed suppurations, but far more rarely than the other two forms. Its pyogenic power is even weaker than that of the albus.

Staphylococcus pyogenes albus, the white coccus, acts like the aureus, but is more feeble in power. When this organism is found upon and in the skin, it is called staphylococcus epidermis albus, an organism which is the cause of stitch abscesses.

Streptococcus pyogenes is found in spreading suppurations and in very acute abscesses. About 16% of acute abscesses contain streptococci. It is easily killed by boiling, and can be destroyed by carbolic acid and by corrosive sublimate. The streptococcus of erysipelas is thought to be identical with the streptococcus pyogenes, but their difference in action is believed to be due to difference in virulence induced by external conditions and by the state of the tissues of the host. The coccus of erysipelas is larger than the ordinary form of streptococcus pyogenes, and infection takes place through a wound, often a very trivial one, or through a mucous membrane. The organism multiplies in the small lymph channels. The streptococcus may cause suppuration in erysipelas, mixed infection not being necessary to cause pus to form.

The gonococcus of Neisser is found both inside and outside of pus cells and mucous cells. The gonococci cannot be cultivated upon ordinary media, but grow best upon human-blood serum. Gonococci stain easily and are readily decolorized by Gram’s method.

The bacillus coli communis, or the bacillus of Escherich, is invariably found in the fÆces. It is believed by many observers to be the cause of appendicitis, peritonitis, and abscesses about the intestine. In cases of appendicitis we can rarely get a pure culture of Escherich’s bacillus, but usually find also streptococci and staphylococci.

The bacillus of typhoid fever (Eberth’s bacillus) is responsible for some cases of gangrene, for some of embolism and for not a few bone and joint diseases.

The bacillus tuberculosis (Koch’s bacillus), the cause of all tubercular processes, is met with especially in dusty air which contains the dried sputum of victims of tuberculosis. This infected air is the chief means of its transmission, though it may be conveyed by the milk of tubercular cows and by the meat of tubercular animals. Wounds may open a gateway for infection.

The bacillus tetani (Nicolaier’s bacillus), an aerobic organism, is found especially in the soil of gardens, in the dust of old buildings, in street dirt, and in the sweepings of stables. Spores develop at the ends of these bacilli. This organism is capable of producing toxins of deadly power. Its spores are hard to kill.