The discussion of the physiology of bacteria in the preceding chapters has shown that a number of environmental factors must be properly correlated in order that a given organism may thrive. Conversely, it can be stated that any one of these environmental factors may be so varied that the organism will be more or less injured, may even be destroyed by such variation. It has been the thorough study of the above-mentioned relationships which has led to practical methods for destroying bacteria, for removing them or preventing their growth when such procedures become necessary.
The process of killing all the living organisms or of removing them completely is spoken of as disinfection or as sterilization, according to circumstances. Thus the latter term is applied largely in the laboratory, while the former more generally in practice outside the laboratory. So also disinfection is most commonly done with chemical agents and sterilization by physical means, though exceptions are numerous. The original idea of disinfection was the destruction of “infective” organisms, that is, organisms producing disease in man or animals. A wider knowledge of bacteriology has led to the application of the term to the destruction of other organisms as well. Thus the cheese-maker “disinfects” his curing rooms to prevent abnormal ripening of cheese, and the dairy-worker “disinfects” his premises to avoid bad flavors, abnormal changes in the butter or milk. Sterilization is more commonly applied to relatively small objects and disinfection to larger ones. Thus in the laboratory, instruments, glassware, apparatus, etc., are “sterilized” while desks, walls and floors are “disinfected.” The surgeon “sterilizes” his instruments, but “disinfects” his operating table and room. The dairy-workers mentioned above sterilize their apparatus, pails, milk bottles, etc. Evidently the object of the two processes is the same, removing or destroying living organisms, the name to be applied is largely a question of usage and circumstances. Any agent which is used to destroy microörganisms is called a “disinfectant.” Material freed from living organisms is “sterile.”
The process of preventing the growth of organisms without reference to whether they are killed or removed is spoken of as “antisepsis,” and the agent as an antiseptic. Hence a mildly applied “disinfectant” becomes an “antiseptic,” though it does not necessarily follow that an “antiseptic” may become a disinfectant when used abundantly. Thus strong sugar solutions prevent the development of many organisms, though they do not necessarily kill them.
Asepsis is a term which is restricted almost entirely to surgical operations and implies the taking of such precautions that foreign organisms are kept out of the field of operation. Such an operation is an aseptic one, or performed aseptically.
A “deodorant or deodorizer” is used to destroy or remove an odor and does not necessarily have either antiseptic or disinfectant properties.
The agents which are used for the above-described processes may be conveniently divided into physical agents and chemical agents.
1. Drying.—This is doubtless the oldest method for preventing the growth of organisms, and the one which is used on the greatest amount of material at the present time. A very large percentage of commercial products is preserved and transported intact because the substances are kept free from moisture. In the laboratory many materials which are used as food for bacteria (see Chapter XVI) “keep” because they are dry. Nevertheless, drying should be considered as an antiseptic rather than as a disinfectant process. While it is true that the complete removal of water would result in the death of all organisms this necessitates a high temperature, in itself destructive, and does not occur in practice. Further, though many pathogenic bacteria are killed by drying, many more, including the spore formers, are not. Hence drying alone is not a practical method of disinfecting.
2. Heat.—The use of heat in some form is one of the very best means for destroying bacteria. It may be made use of by combustion, or burning, as direct exposure to the open flame, as dry heat (hot air), or as moist heat (boiling water or steam). Very frequently in veterinary practice, especially in the country, occasionally under other conditions, the infected material is best burned. This method is thoroughly effective and frequently the cheapest in the end. Wherever there are no valid objections it should be used. Exposure to the open flame is largely a laboratory procedure to sterilize small metallic instruments and even small pieces of glassware. It is an excellent procedure in postmortem examinations to burn off the surface of the body or of an organ when it is desired to obtain bacteria from the interior free from contamination with surface organisms.
Dry Heat.—Dry heat is not nearly so effective as moist heat as a sterilizing agent. The temperature must be higher and continued longer to accomplish the same result. Thus a dry heat of 150° for thirty minutes is no more efficient than steam under pressure at 115° for fifteen minutes. Various forms of hot-air sterilizers are made for laboratory purposes (Fig. 77). On account of the greater length of time required for sterilization their use is more and more restricted to objects which must be used dry, as in blood and serum work, for example. In practice the use of hot air in disinfecting plants is now largely restricted to objects which might be injured by steam, as leather goods, furs, and certain articles of furniture, but even here chemical agents are more frequently used.
Moist Heat.—Moist heat may be applied either by boiling in water or by the use of steam at air pressure, or, for rapid work and on substances that would not be injured, by steam under pressure. Boiling is perhaps the best household method for disinfecting all material which can be so treated. The method is simple, can always be made use of, and is universally understood. It must be remembered that all pathogenic organisms, even their spores, are destroyed by a few minutes’ boiling. The process may be applied to more resistant organisms, such as are met with in canning vegetables, though the boiling must be continued for several hours, or what is better, repeated on several different days. This latter process, known as “discontinuous sterilization,” or “tyndallization,” must also be applied to substances which would be injured or changed in composition by too long-continued heating, such as gelatin, milk, and certain sugars. In the laboratory such materials are boiled or subjected to steaming steam for half an hour on each of three successive days. In canning vegetables the boiling should be from one to two hours each day. The principle involved is that the first boiling destroys the growing cells, but not all spores. Some of the latter germinate by the next day and are then killed by the second boiling and the remainder develop and are killed on the third day. Occasionally a fourth boiling is necessary. It is also true that repeated heating and cooling is more destructive to bacteria than continuous heating for the same length of time, but the development of the spores is the more important factor. Discontinuous heating may also be used at temperatures below the boiling-point for the sterilization of fluids like blood serum which would be coagulated by boiling. In this case the material is heated at 55° to 56° for one hour, but on each of seven to ten successive days. The intermittent heating and cooling is of the same importance as the development of the spores in this case. (Better results are secured with such substances by collecting them aseptically in the first place.)
Steam.—Steam is one of the most commonly employed agents for sterilization and disinfection. It is used either as “streaming steam” at air pressure or confined under pressure so that the temperature is raised. For almost all purposes where boiling is applicable streaming steam may be substituted. It is just as efficient and frequently more easily applied. The principle of the numerous forms of “steam sterilizers” (Fig. 78) is essentially the same. There is a receptacle for a relatively small quantity of water and means for conducting the steam generated by boiling this water to the objects to be treated, which are usually placed immediately above the water. Surgical instruments may be most conveniently sterilized by boiling or by steaming in especially constructed instrument sterilizers. If boiled, the addition of carbonate of soda, about 1 per cent., usually prevents injury.
Steam under pressure affords a much more rapid and certain method of destroying organisms. Fifteen to twenty pounds pressure corresponding to temperatures of 121° to 125° is commonly used. Variations depend on the bulk and nature of the material. Apparatus for this purpose may now be obtained from sizes as small as one or two gallons up to huge structures which will take one or two truckloads of material (Figs. 79–91). The latter type is in common use in canning factories, dairy plants, hospitals, public institutions, municipal and governmental disinfecting stations. Very frequently there is an apparatus attached for producing a vacuum, both to exhaust the air before sterilizing, so that the steam penetrates much more quickly and thoroughly and for removing the vapor after sterilizing, thus hastening the drying out of the material disinfected.
The smaller types of pressure sterilizers are called “autoclaves” and have become indispensable in laboratory work. Fifteen pounds pressure maintained for fifteen minutes is commonly sufficient for a few small objects. For larger masses much longer time is needed. The author found that in an autoclave of the type shown in Fig. 81 it required ten minutes for 500 cc. of water at 15 pounds pressure to reach a temperature of 100°, starting at room temperatures, 20° to 25°. Autoclaves may be used as simple steam sterilizers by leaving the escape valves open so that the steam is not confined, hence they have largely replaced the latter.18
A process closely akin to sterilization by heat is pasteurization. This means the heating of material at a temperature and for a time which will destroy the actively growing bacteria but not the spores. The methods for doing this vary but are essentially two in principle. 1. The material in small quantities in suitable containers (bottles) is placed in the apparatus; the temperature is raised to 60° to 65° and maintained for twenty to thirty minutes and then the whole is cooled (beer, wine, grape juice, bottled milk) (Figs. 92, 93 and 94).
2. Pasteurizing machines are used and the fluid flows through continuously. In one type the temperature is raised to 60° and by “retarders” is kept at this temperature for twenty to thirty minutes (Figs. 95 to 98). In another type the temperature is raised to as high as 85° for a few seconds only, “flash process” (Fig. 99), and then the material is rapidly cooled. It is certain that all pathogenic microörganisms, except the very few spore formers in that stage, are killed by proper pasteurization. The process is largely employed in the fermentation and dairy industries.
3. Cold.—That cold is an excellent antiseptic is illustrated by the general use of refrigerators and “cold storage.” Numerous experiments have shown that although many pathogenic organisms of a given kind are killed by temperatures below freezing, not all of the same kind are, and many kinds are only slightly affected. Hence cold cannot be considered a practical means for disinfection.
4. Light.—It has been stated (p. 75) that light is destructive to bacteria, and the advisability of having well-lighted habitations for men and animals has been mentioned. The practice of “sunning” bedclothing, hangings and other large articles which can scarcely be disinfected in a more convenient way is the usual method of employing this agent. Drying and the action of the oxygen of the air assist the process to some extent. Undoubtedly large numbers of pathogenic organisms are destroyed under natural conditions by the combined effects of drying, direct sunlight and oxidation, but it should not be forgotten that a very slight protection will prevent the action of light (Figs. 100 and 101).
5. Osmotic Pressure.—Increase in the concentration of substances in solution is in practical use as an antiseptic procedure. Various kinds of “sugar preserves,” salt meats and condensed milk are illustrations. It must be remembered that a similar increase in concentration occurs when many substances are dried, and is probably as valuable in the preservative action as the loss of water. That the process cannot be depended on to kill even pathogenic organisms is shown by finding living tubercle bacilli in condensed milk. The placing of bacteria in water or in salt solution in order to have them die and disintegrate (greatly aided by vigorous shaking in a shaking machine) (“autolysis,” p. 126) is a laboratory procedure to obtain cell constituents. It is not a practical method of disinfection, however.
6. Electricity.—Electricity, though not in itself injurious to bacteria, is used as an indirect means for destroying bacteria in a practical way. This is done by electrical production of some substance which is destructive to bacteria as in ozone water purification (Petrograd, Florence, and elsewhere), or the use of ultra-violet rays for the same purpose (Marseilles, Paris) and for treatment of certain disease conditions. Electricity might be used as a source of heat for disinfecting purposes should its cheapness justify it. It has also been used in the preservation of meats to hasten the penetration of the salt and thus reduce the time of pickling. Electrolyzed sea water has been tried as a means of flushing and disinfecting streets, but it is very doubtful if the added expense is justified by any increased benefit. A number of electric devices have been put forth for various sterilizing and disinfecting purposes and doubtless will continue to be, but everyone should be carefully tested before money is invested in it.19
7. Filtration.—Filtration is a process for rendering fluids sterile by passing them through some material which will hold back the bacteria. It is used on a large scale in the purification of water for sanitary or manufacturing reasons (Fig. 103). Air is also rendered “germ free” in some surgical operating rooms, “serum laboratories” and breweries by filtration. In the laboratory it is a very common method of sterilizing liquids which would be injured by any other process. The apparatus consists of a porous cylinder with proper devices for causing the liquid to pass through either by suction (Fig. 104) where the pressure will be only one atmosphere (approximately 15 pounds per square inch), or by the use of compressed air at any desired pressure (Fig. 105). The two main types of porous cylinders (“filter candles,” “bougies”) are the Pasteur-Chamberland (Fig. 106) and the Berkefeld. The former are made of unglazed porcelain of different degrees of fineness, the latter of diatomaceous earth (Fig. 107) The Mandler filter of this same material is now manufactured in the United States and is equal if not superior to the Berkefeld. The designs of complete apparatus are numerous.
8. Burying.—This is a time-honored method of disposing of infected material of all kinds and at first thought might not be considered a means of disinfection. As a matter of fact, under favorable conditions it is an excellent method. The infected material is removed. Pathogenic organisms tend to die out in the soil owing to an unfavorable environment as to temperature and food supply, competition with natural soil organisms for what food there is, and the injurious effects of the products of these organisms. Care must be taken that the burial is done in such a way that the surface soil is not contaminated either directly or by material brought up from below by digging or burrowing animals, insects, worms, or movement of ground water to the surface. Also that the underground water supply which is drawn upon for use by men or animals is not contaminated. Frequently infected material, carcasses of animals, etc., are treated in some way so as to aid the natural process of destruction of the organisms present, especially by the use of certain chemical agents, as quicklime (see p. 158).