The Fundamentals of Bacteriology

CHAPTER XV.
DISINFECTION AND STERILIZATION (Continued).

CHOICE OF AGENT.

The choice of the above-described agents depends on the conditions. Evidently a barn is not to be disinfected in the same way that a test-tube in the laboratory is sterilized. Among the factors to be considered in making a choice are the thing to be disinfected or sterilized, its size and nature, that is, whether it will be injured by the process proposed, cost of the agent, especially when a large amount of material is to be treated. Among the conditions which affect the action of all agents the following should be borne in mind particularly when testing the disinfecting power of chemical agents:

1. The kind of bacterium to be destroyed, since some are more readily killed by a given disinfectant than others, even though no spores are present.

2. The age of the culture. Young bacteria less than twenty-four hours old are usually more readily killed than older ones since the cell wall is more delicate and more easily penetrated, though old growths may be weakened by the accumulation of their products and be more easily destroyed.

3. Presence of spores, since they are much more resistant than the growing cells.

4. Whether the organism is a “good” or “bad” growth, i.e., whether it has grown in a favorable environment and hence is vigorous, or under unfavorable conditions and hence is weak.

5. The number of bacteria present, since with chemical agents the action is one of relative masses.

6. Nature of the substance in which the bacteria are. Metallic salts, especially bichloride of mercury, are precipitated by albuminous substances and if employed at all must be used in several times the ordinary strength. Solids require relatively more of a given solution than liquids.

7. State of the disinfectant, whether solid, liquid or gas, and whether it is ionized or not. Solutions penetrate best and are therefore more quickly active and more efficient.

8. The solvent. Water is the best solvent to use. Strong alcohol (90 per cent. +) diminishes the effect of carbolic acid, formaldehyde and bichloride of mercury. Oil has a similar effect. The action is probably to prevent the penetration of the disinfectant.

9. Strength of solution. The stronger the solution, the more rapid and more certain the action, for the same reason as mentioned under 5. In fact, every disinfectant has a strength below the lethal at which it stimulates bacterial growth.

10. Addition of salts. Common salt favors the action of bichloride of mercury and also of carbolic acid. Other salts may hinder by precipitating the disinfectant.

11. Temperature. Chemical disinfectants, as a rule, follow the general law that chemical action increases with the temperature, up to the point where the heat of itself is sufficient to kill.

12. Time of action. It is scarcely necessary to point out that a certain length of time is necessary for any disinfectant to act. One may touch a red hot stove and not be burned. All the above-mentioned conditions are influenced by the time of action.

STANDARDIZATION OF DISINFECTANTS—“PHENOL COEFFICIENT.”

Many attempts have been made to devise standard methods for testing the relative strengths of disinfectants. The one most widely used in the United States is the so-called “Hygienic Laboratory” method of determining the “phenol coefficient” of the given substance and is a modification of the method originally proposed by Rideal and Walker in England. In this method as proposed by Anderson and McClintic, formerly of the above laboratory, the strengths of the dilution of the disinfectant to be tested which kills a culture of Bacterium typhosum in 2½ minutes is divided by the strength of the dilution of carbolic acid which does the same; and the dilution which kills in 15 minutes is likewise divided by the corresponding dilution of carbolic acid. The two ratios thus obtained are averaged and the result is the “phenol coefficient.” For example

Standard conditions of temperature, age of culture, medium, reaction, etc., and of making the dilutions and transfers are insisted on. Details may be found in the Journal of Infectious Diseases, 1911, 8, p. 1.

This is probably as good a method as any for arriving at the relative strengths of disinfectants and in the hands of any given worker concordant results in comparative tests can usually be attained. Experience has shown that the results obtained by different workers with the same disinfectant may be decidedly at variance. This is to be expected from a knowledge of the factors affecting the action of disinfectants above stated and from the known specific action of certain disinfectants on certain organisms (compare anilin dyes, p. 162).

It seems that the only sure way to test the action of such a substance is to try it out in the way it is to be used. It is scarcely wise to adopt the “phenol coefficient” method as a legal standard method as some states have done.

PRACTICAL STERILIZATION AND DISINFECTION.

The methods for sterilizing in the laboratory have been discussed and will be referred to again in the next chapter.

In practical disinfection it is a good plan always to proceed as though spores were present even if the organism is known. Hence use an abundance of the agent and apply it as long as practicable. Also it is best to secure the chemical substances used as such and not depend on patented mixtures purporting to contain them. As a rule the latter are more expensive in proportion to the results secured.

Surgical instruments may be sterilized by boiling in water for fifteen minutes, provided they are clean, as they should be. If dried blood, pus, mucus, etc., are adherent, which should never be the case, they should be boiled one-half hour. The addition of sodium carbonate (0.5 to 1 per cent.) prevents rusting. Surgeons’ sterilizers are to be had at reasonable prices and are very convenient. Whether the instruments are boiled or subjected to streaming steam depends on whether the supporting tray is covered with water or not. The author finds it a good plan to keep the needles of hypodermic syringes in a small wire basket in an oil bath. The oil may be heated to 150° to 200° and the needles sterilized in a very few minutes. The oil also prevents rusting.

Rooms, offices and all spaces which may be readily made practically gas-tight are best disinfected by means of formaldehyde by any of the methods above described (Figs. 111 and 112).

Stables and Barnyards (Mohler): “A preliminary cleaning up of all litter is advisable together with the scraping of the floor, mangers and walls of the stable with hoes and the removal of all dust and filth. All this material should be burned since it probably contains the infective agent. Heat may be applied to the surfaces, including barnyard, by means of a ‘cyclone oil burner.’ When such burning is impracticable, the walls may be disinfected with one of the following:

1. 1. Whitewash 1 gallon + chloride of lime 6 ounces.
2. 2. Whitewash 1 gallon + crude carbolic acid 7 ounces.
3. 3. Whitewash 1 gallon + formalin 4 ounces.

The same may be applied with brushes or, more rapidly, sprayed on with a pump; the surface soil of the yard and surroundings should be removed to a depth of 5 or 6 inches, placed in a heap and thoroughly mixed with quicklime. The fresh surface of soil thus exposed may be sprinkled with a solution of a chemical disinfectant as above described.

“Portions of walls and ceiling not readily accessible may be disinfected by chlorine gas liberated from chloride of lime by crude carbolic acid. This is accomplished by making a cone of 5 or 6 pounds of chloride of lime in the top of which a deep crater is made for the placement of from 1 to 2 pints of crude carbolic acid. The edge of the crater is thereupon pushed into the fluid, when a lively reaction follows. Owing to the heat generated, it is advisable to place the chloride of lime in an iron crucible (pot), and to have nothing inflammable within a radius of two feet. The number and location of these cones of chloride of lime depend on the size and structure of the building to be disinfected. As a rule it may be stated that chlorine gas liberated from the above sized cone will be sufficient for disinfecting 5200 cubic feet of air space.”

Liquid manure, leachings, etc., where collected are thoroughly disinfected by chloride of lime applied in the proportion of 2 parts to 1000 of fluid.

Vehicles may be thoroughly washed with 2 per cent. formalin solution, or if closed space is available, subjected to formaldehyde gas disinfection, after cushions, hangings, etc., have been removed and washed with the disinfectant.

Harness, brushes, combs should be washed with a solution of formalin, carbolic acid, or creolin as given under these topics.

Washable articles should be boiled, dropped into disinfectant, solutions as soon as soiled, and then boiled or steamed.

Unwashable articles—burn all possible. Use formaldehyde gas method in a closed receptacle (Fig. 113).

Stock cars—the method described for stables is applicable here.

Animals, large and small, may have the coat and surface of the body disinfected by washing with 1 to 1000 bichloride or strong hot soapsuds to which carbolic acid has been added to make a 5 per cent. solution; they should then be given a good warm bath.

Frequently time and money are saved by a combination of steam and formaldehyde disinfection. This is a regular practice in municipal and quarantine disinfection (Fig. 114).

Persons engaged in disinfection work should wear rubber boots, coats and caps which should be washed in a disinfectant solution and the change to ordinary clothing made in a special room so that no infective material will be taken away.