Popular Science Monthly/Volume 17/October 1880/On the Destruction of Infectious Germs



THE theory that contagious diseases as well as putrefaction and fermentation are developed and propagated by the agency of organisms allied to the bacteria has been widely accepted, and is supported by the results of recent investigations. It becomes, then, of paramount importance to ascertain the most efficacious means of destroying these organisms.

With this object in view, these researches into the conditions under which bacteria may be destroyed have taken three directions: 1. To test an observation made by Ernst Baumann, that the putrefactive organisms in the course of their action develop carbolic acid, a deadly poison to them, and to inquire whether there are not other poisons to bacteria developed in a similar manner; 2. Investigations prosecuted during the prevalence of the plague to ascertain whether a dry disinfection of clothing and goods could be made effective wholly to destroy the infectious organisms; 3. Having transplanted active infectious organisms from one substance to another to which they are suited, to arrest them in the most rapid stage of their development, destroy them, or cause them to perish.

In order to make the experiments of real value, a sure means must be found of knowing whether the organisms are alive or dead; they may seem dead when they are only passive. The only unfailing test is afforded by the reproductive faculty: when reproduction ceases, and can not be excited, the organisms may be considered dead.

Particular investigators have doubted whether it is possible wholly to destroy these lower organisms. Naegeli[1] says it can not be fully done without the aid of heat, and even heat is not always equally effective. They are generally more easily destroyed by heat when moist than when dry, but even a boiling heat will not destroy some of them when they are in fluids of a neutral reaction. The more acid the reaction, the less is the degree of heat that is required. The degree of heat required to destroy the germs of infectious diseases is believed to be greater than it is practicable to apply by the dry process to clothing and similar materials. The capacity of many of the organisms to reproduce may, indeed, be destroyed by a more moderate temperature, but a question remains concerning the germs or spores which had been taken up into the materials and were carried away with them. These are believed to have some kind of a coating which enables them to resist what destroys the parent organisms.

In order to test the value of the dry process as applied to infected clothing, pieces of different clothing materials were impregnated with strong putrefying and bacteria-bearing fluids, then dried slowly, and kept for a long time without protection against external influences. Whenever the smallest piece of one of these materials was put into a suitable fluid, the perturbation invariably took place which is the sure sign of the active multiplication of bacteria. Clothing which had not been impregnated did not excite this perturbation, or only in an insignificant degree. Specimens of the defiled clothing which were placed in a similar solution after they had been exposed for five minutes to a temperature of from 125° to 150° C, or for one or two minutes to a higher temperature, produced no change. The capacity of the bacteria to resist heat varies widely among the different species, and appears to depend largely on the faculty of developing spores. The individuals are killed, but the spores remain vital. The increase of any one kind is limited by the presence of other kinds, with which a struggle for existence has to be maintained.

No increase of bacteria takes place without the presence of a suitable substance to support them. The most favorable of non-nitrogeneous substances is sugar; among nitrogeneous substances the most favorable are the albuminoids; among mineral matters, potash, phosphorus, magnesia, and sulphur. If the supporting substance, even though it is needed in only a minute quantity, is consumed, or if it is present in great excess, a pause in the development, but not the death of the bacteria, takes place. A similar effect is produced by taking away the water, but, when the water is restored, an increase of life again takes place.

The practical object of disinfection should be to go beyond securing a suspension of animation of the bacteria, and to seek to destroy the vitality of the spores. Neither years of dryness, nor months of exposure in foul water, nor repeated drying and moistening, will injure the fertility of these germs.

An excess of water produces a similar effect with desiccation upon the vital conditions of the bacteria. A great dilution of the supporting fluid by the infusion of pure water will in a short time produce a suspension of the process of decomposition. Privation of light has no effect. The operation of electricity has not been enough observed to justify the drawing of any conclusion. The effect of the privation of air has not been fully determined. It was once thought that the development of bacteria could be hindered by the removal of oxygen, but this is doubtful. Oxygen greatly speeds the development, but it can take place without it. Bacteria are not developed in nitrogen, hydrogen, carbonic oxide, carbonic acid, nitrous oxide, and illuminating gas.

The substances which are fatal to the life of the bacteria next demand attention. Among these, the concentrated mineral acids, iodine, bromine, chlorine, the sulphates of copper and zinc, corrosive sublimate, benzoic acid and its salts, salicylic and metasalicylic acids, quinia, many aromatic substances, and alcohol, have long been known as such. Carbolic acid is the highest in repute among these poisons; and it is an interesting fact that Mr. E. Baumann has discovered this very substance among the products of the bacterian fermentation to which it is so fatal. Alcohol is another substance similarly associated. The discovery of the curious relations of these two substances gives a new light upon the cause of the spontaneous destruction of bacteria in strongly fermenting fluids, and encourages us to look for other substances having a similar origin and a like action. As evidence of the possession of such properties by any substance, we should require—

1. That substances favorable to the development of bacteria should remain free from them when the substance to be tested is added to them.

2. That active bacteria, when transplanted into a supporting mixture to which a substance supposed to be poisonous to them has been added, should cease to propagate themselves and die out. This may be called the aseptic test.

3. That, when the supposed poison is introduced into a solution swarming with bacteria, all living examples should be killed. This may be called the antiseptic test.

Various aromatic substances, the products of fermentation, were added to a mixture of water and chopped meat, at a temperature of 35° C They proved efficacious in preventing, suspending, or wholly stopping decomposition in the following order, according to the strength of their working:

1. As Preventives of Decomposition:

Indol in a proportion of 1 :1,000 of the mixture.
Kresol "" 2 :1,000 "
Phenylacetic acid "" 2·5 :1,000 "
Carbolic acid "" 5 :1,000 "

(The working of scatol and hydrocinnamic acid could not be satisfactorily ascertained, on account of the difficulty of dissolving them in water.)

2. As Aseptics—killing transplanted organisms by poisoning the supporting fluid:

Scatol in a proportion of 0·4 :1,000 of the mixture.
Hydrocinnamic acid "" 0·6 :1,000 "
Indol "" 0·6 :1,000 "
Kresol "" 0·8 1,000 "
Phenylacetic acid "" 1·2 1,000 "
Carbolic acid "" 5 :1,000 "

3. As Antiseptics—wholly destroying all living bacteria:

Scatol, in the proportion of 0·5: 1,000 of the mixture, in twenty-four hours. Hydrocinnamic acid, in a proportion of 0·8: 1,000 of the mixture, in twenty-four hours.

Phenylacetic acid, in a saturated solution (1:400), immediately.
Indol, in a saturated solution (1:900), in twenty-four hours.
Kresol, in the proportion of 5:1,000 of the mixture, in twenty-four hours.
Carbolic acid, in the proportion of 20:1,000 of the mixture, immediately.

Two points strike us in this review: first, the difference in the amount of poison required to produce the aseptic and the antiseptic effect; again, it is curious that carbolic acid, the favorite antiseptic, appears to be the weakest on the list. It is at the same time one of the most soluble, while scatol, the most difficult of solution, is the strongest.

If we add the substances we have been examining to a saccharine solution exposed to fermentation, a slackening of the fermenting process will take place, and the different substances will, as before, exhibit their power to delay the process in the following order: scatol, hydrocinnamic acid, indol, phenylacetic acid, kresol, carbolic acid.

These facts seem to justify us in looking for specific disinfectants and prophylactics among the aromatic products of chemical decomposition. They also give a strong air of plausibility to the theory that the bacteria produce, through the chemical changes of which they are the direct cause, the most effective substances that can be used to destroy them. The idea is logically deducible from this theory that the germs of disease finally produce their own destruction by the operation of their growth and development, and helps us to comprehend the cyclical course which is characteristic of most infectious diseases.

  1. Die niederen Filze in ihren Beziehungen zu den Iufektionskrankheiten und der Gesimdheitspflege, s. 201.