CHAPTER XIV.
DISINFECTION AND STERILIZATION (Continued).
CHEMICAL AGENTS.
A very large number of chemical substances might be used for destroying bacteria or preventing their growth either through direct injurious action or by the effect of concentration. Those which are practically useful are relatively few, though this is one of the commonest methods of disinfecting and the word “disinfectant” is frequently wrongly restricted to chemical agents.
Chemical agents act on bacteria in a variety of ways. Most commonly there is direct union of the chemical with the protoplasm of the cell and consequent injury. Some times the chemical is first precipitated on the surface of the cell without penetrating at once. If removed soon enough, the organism is not destroyed. This is true of bichloride of mercury and formaldehyde. If bacteria treated with these agents in injurious strength be washed with ammonia or ammonium sulphate, even after a time which would otherwise result in their failure to grow, they will develop. Some chemicals change the reaction of the material in a direction unfavorable to growth, and if the change is enough, may even kill the bacteria. Some agents remove a chemical substance necessary to the growth of the organism and hence inhibit it. Such actions are mainly preventive (antiseptic) and become disinfectant only after a long time.
ELEMENTS.
Oxygen.—Oxygen as it occurs in the air is probably not injurious to living bacteria but aids them with the exception of the anaërobes. In the nascent state especially as liberated from ozone (O3) hydrogen peroxide (H2O2) and hypochlorites (Ca(ClO)2) it is strongly bactericidal.
Chlorine.—Chlorine is actively disinfectant and is coming into use for sterilizing water on a large scale in municipal plants (Fig. 108).
Iodine finds extended use in aseptic surgical operations and antiseptic dressings. Bromine, mercury, silver, gold, nickel, zinc and copper are markedly germicidal in the elemental state but are not practical.
COMPOUNDS.
Calcium Oxide.—Calcium oxide (CaO), quick lime, is an excellent disinfectant for stables, yards, outhouses, etc., where it is used in the freshly slaked condition as “white wash;” also to disinfect carcasses to be buried. It is very efficient against the typhoid bacillus in water, where it is much used to assist in the softening.
Chloride of Lime.—Chloride of lime, bleaching powder, which consists of calcium hypochlorite, the active agent, and chloride and some unchanged quicklime is one of the most useful disinfectants. It is employed to sterilize water for drinking purposes on a large scale and to disinfect sewage plant effluents. A 5 per cent. solution is the proper strength for ordinary disinfection. Only a supply which is fresh or has been kept in air-tight containers should be used, as it rapidly loses strength on exposure to the air. The active agent is nascent oxygen liberated from the decomposition of the hypochlorite.
Sodium Hypochlorite.—Sodium hypochlorite prepared by the electrolysis of common salt has been used to some extent.
Bichloride of Mercury.—Bichloride of mercury, mercuric chloride, corrosive sublimate (HgCl2), is the strongest of all disinfectants under proper conditions. It is also extremely poisonous to men and animals and great care is necessary in its use. It is precipitated by albuminous substances and attacks metallic objects, hence should not be used in the presence of these classes of substances.
It is used in a strength of one part HgCl2 to 1000 of water for general disinfection. Ammonium chloride or sodium chloride, common salt, in quantities equal to the bichloride, or citric acid in one-half of the amount should be added in making large quantities of solution or for use with albuminous fluids to prevent precipitation of the mercury (Fig. 109).
None of the other metallic salts are of value as practical disinfectants aside from their use in surgical practice. In this latter class come boric acid, silver nitrate, potassium permanganate. The strong mineral acids and alkalies are, of course, destructive to bacteria, but their corrosive effect excludes them from practical use, except that “lye washes” are of value in cleaning floors and rough wood-work, but even here better disinfection can be done more easily and safely.
ORGANIC COMPOUNDS.
Carbolic Acid or Phenol.—Carbolic acid or phenol (C6H5 OH) is one of the commonest agents in this class. It is used mostly in 5 per cent. solution as a disinfectant and in 0.5 per cent. solution as an antiseptic. For use in large quantities the crude is much cheaper and, according to some experimenters, even more active than the pure acid, owing to the cresols it contains. The crude acid is commonly mixed with an equal volume of commercial sulphuric acid and the mixture is added to enough water to make a 5 per cent. dilution, which is stronger than either of the ingredients alone in 5 per cent. solution.
Cresols.—The cresols (C6H4CH3OH, ortho, meta and para), coal-tar derivatives, as phenol, are apparently more powerful disinfectants. A great number of preparations containing them have been put on the market. Creolin is one which is very much used in veterinary practice and forms a milky fluid with water, while lysol forms a clear frothy liquid owing to the presence of soap. Both of these appear to be more active than carbolic acid and are less poisonous and more agreeable to use. They are used in 2 to 5 per cent. solution.
Alcohol.—Ordinary (ethyl) alcohol (C2H5OH) is largely used as a preservative, also as a disinfectant for the body surface, hands, and arms. Experiments show that alcohol of 70 per cent. strength is most strongly bactericidal and that absolute alcohol is very slightly so.
Soap.—Experimenters have obtained many conflicting results with soaps when tested on different organisms, as is to be expected from the great variations in this article. Miss Vera McCoy in the author’s laboratory carried out experiments with nine commercial soaps—Ivory, Naphtha, Packer’s Tar, Grandpa’s Tar, Balsam Peru, A. D. S. Carbolic, German Green, Dutch Cleanser, Sapolio—and obtained abundant growth from spores of Bacillus anthracis, from Bacterium coli and from Staphylococcus pyogenes aureus in all cases even when the organisms had been exposed twenty-four hours in 5 per cent. solutions. From these results and from the wide variations reported in the literature it is clear that soap solutions alone cannot be depended on as disinfectants. Medicated soaps do not appear to offer any advantages in this respect. The amount of the disinfectant which goes into solution when the soap is dissolved is too small to have any effect.
Formaldehyde.—Formaldehyde (HCHO) is perhaps the most largely used chemical disinfectant at the present time. The substance is a gas but occurs most commonly in commerce as a watery solution containing approximately 40 per cent. of the gas. This solution is variously known as formalin, formol, and formaldehyde solution. The first two names are patented and the substance under these names usually costs more. It is used in the gaseous form for disinfecting closed spaces of all kinds to the exclusion of most other means today. A great many types of formalin generators have been devised. The gas has little power of penetration and all material to be reached should be exposed as much as possible. The dry gas is almost ineffective, so that the objects must be moistened or vapor generated along with the gas. A common method in use is to avoid expensive generators by pouring the formaldehyde solution on permanganate of potash crystals placed in a vessel removed from inflammable objects on account of the heat developed which occasionally sets the gas on fire. The formalin is used in amounts varying from 20 to 32 ounces to 8½ to 13 ounces of permanganate to each 1000 cubic feet of space. This method is expensive since one pint (16 ounces) of formalin is sufficient for each 1000 cubic feet, and since the permanganate is an added expense. Dr. Dixon, Commissioner of Health of Pennsylvania, recommends the following mixture to replace the permanganate, claiming that it works more rapidly and is less expensive and just as efficient:
- 1. Sodium bichromate, ten ounces.
- 2. Saturated solution of formaldehyde, sixteen ounces.
- 3. Common sulphuric acid, one and a half ounces.
Two and three are mixed together and when cool are poured on the bichromate which is placed in an earthenware jar of a volume about ten times the quantity of fluid used. The quantities given are for each 1000 cubic feet of space.
A very simple method is to cause the formalin, diluted about twice with water to furnish moisture enough, to drop by means of a regulated “separator funnel” on a heated iron plate. The dropping should be so regulated that each drop is vaporized as it falls. The plate must have raised edges, pan-shaped, to prevent the drops rolling off when they first strike the plate. Formaldehyde has no corrosive (except on iron) or bleaching action, and is the most nearly ideal closed space disinfectant today. In disinfecting stations it is made use of in closed sterilizers such as were described under steam disinfection particularly in connection with vacuum apparatus. It is also used in solution as a preservative and as a disinfectant. The commonest strength is 2 or 3 per cent. of formalin or 0.8 to 1.2 per cent. of the formaldehyde gas. As an antiseptic it is efficient in dilutions as high as 1 to 2000 of the gas. It is very irritant to mucous membranes of most individuals.
Anilin Dyes.—Some of the anilin dyes show remarkable selective disinfectant and antiseptic action on certain kinds of bacteria with little effect on others. This has been well shown by Churchman in his work on Gentian Violet. This dye inhibits the growth of Gram positive organisms up to a dilution of one part in 300,000 while for Gram negative organisms it is without effect even in saturated solution. This is nicely shown in the accompanying illustration. This inhibiting effect of anilin dyes is taken advantage of in several methods of isolating bacteria (Chapter XVIII).
In addition to the above-discussed disinfectants a large number of substances, particularly organic, are used in medicine, surgery, dentistry, etc., as more or less strong antiseptics, and the list is a constantly lengthening one.
In the laboratory chloroform, H2O2, ether and other volatile or easily decomposable substances have been used to sterilize liquids which could not be treated by heat or by filtration. The agent is removed either by slow evaporation or by exhausting the fluid with an air pump. The method is not very satisfactory, nor is absolute sterilization easily accomplished. It is much better to secure such liquids aseptically where possible.