Antiseptics in Dentistry

Their Use and Limitations

BY ROBERT A. STEVENSON, D.D.S., R. Ph.

Dental Therapy Editor

 

                No branch of bacteriology has been more fruitful in practical application than that which deals with the factors which bring about the destruction of microorganisms.

The agents which affect bacteria injuriously are many, and are both physical and chemical in nature.

When a procedure completely destroys bacterial life it is spoken of as sterilization or disinfection, the term disinfection being employed more especially to designate the use of chemical agents. When the procedure destroys vegetative forms only, leaving the mores resistant spores uninjured, it is spoken of as “incomplete sterilization.” When an agent, on the other hand, does not actually kill the micro-organisms, but merely inhibits their growth and multiplication, it is spoken of as an antiseptic.

A disinfectant is accordingly only intended to act for a short time, for if the infected matter be once rendered sterile it can only become dangerous by being again contaminated. A would even though completely disinfected may become contaminated again very easily and an antiseptic may be required to prevent further growth of microbes. Many substances are disinfectant in large quantities and antiseptic in more dilute solutions, but others are too weak to disinfect thoroughly though they retard the growth of pathogenic organisms, and still others may be employed to disinfect but are unsuitable for use as antiseptics, either because they are too poisonous to be applied for a sufficient time, or because they lose their activity on contact with living matter (e.g., oxidizing disinfectants).

A very large number of substances possess disinfectant properties, that is, are capable of destroying microbes when they can be applied in sufficient quantity. They have no specific action on the microbes, however, but act as general protoplasm poisons destroying living tissue of all kinds whenever they come in contact with it. On the other hand, drugs such as strychnine, which act on specialized parts of the vertebrate organism and have less effect on the less differentiated tissues, are equally harmless to the undifferentiated protoplasm of the microbes. It is of importance to note that the ordinary antiseptics do not act more strongly on microbes than on the tissues in which they are embedded or the phagocytes with which the organism is combating the infection.

Death of Organisms Entails Death of Tissue

The destruction of the septic organisms in a wounded surface entails the destruction of the surrounding cells also. Thus disinfection can only be carried out in a part in which the superficial cells are not of vital importance and may be restored by new growth. It is therefore impossible to disinfect the tissue of the body as a whole because a drug circulating in the blood in sufficient quantity to destroy the bacteria in the body would be equally detrimental to the organs in which they are embedded. Unless a drug has a specific affinity for the parasite, much greater than that for the tissues of the host, it can only be used where the parasite can be overwhelmed by a massive dose, and this at the expense of the neighboring tissues. The problem is not insoluble, however, but requires the discovery of a reagent which shall differentiate between the protoplasm of bacteria and that of man, in the same way as chemical reagents differentiate between such bodies as potassium and sodium; the two forms of protoplasm of course differ from each other much less than the metals. Quite recently some progress has been made in this quest, for Browning and his associates state that certain basic substances, notably Flavine and Acriflavine (dismino-methyl-acridinium chloride), act more strongly as disinfectants in serum that in water and kill bacteria in a concentration 400 times lower than that required to interfere seriously with phagocytosis. Similarly, Morgenroth has shown that some alkaloidal substances of the quinine series possess a selective action on the pneumococcus, which they are able to destroy in the mouse without injury to the host. This use of basic substances seems to be promising, for there is more probability of their being freed in the body fluids and thus penetrating more readily into the organisms than is possible for acidic substances. Recent work also tends to show that some drugs are more potant against certain bacteria, others against other species, so that an imperfect specificity seems to be present. Thus far, however, the promising results of test-tube experiments have not been confirmed when put to the test in infections in living animals. There is, therefore, hope that the limitations set to the use of disinfectants may have to be modified, but further experience is necessary. In the meantime they hold for all the disinfectants and antiseptics which are in general use.

Antiseptics Oxidize or Coagulate Proteins

The antiseptics and disinfectants act upon most forms of living matter, and in many instances their effects are obviously due to their possessing powers of oxidizing or of coagulating proteins. In other instances their destructive action is not so open to explanation. And the amount of destruction induced varies with the degree to which the poison penetrates the tissues to which it is applied. For example, mercuric chloride diffuses deeply into tissues brought in contact with it and causes wide destruction, while the oxidizing disinfectants lose their efficacy on meeting proteins and thus affect only the most superficial cells. If microbes were confined to the surface, the latter would be sufficient for their destruction, but in order to disinfect a wound it is necessary to penetrate more deeply and thus efficient disinfection implies a certain amount of destruction of the tissues in which the microbes are harbored. This local destruction of cells and nervous structures induces pain and irritation and all efficient disinfectants are irritants. Their action as irritants arises from the same qualities  as their disinfectant power, namely, from their general toxicity to living matter, and it is impossible to dissociate the one from the other and to produce nonirritant effective disinfectants.

When a surface has been poisoned by means of disinfectants, it heals less quickly, and this has led to the more sparing use of antiseptics and to the development of the aseptic method, in which organisms are excluded instead of being admitted and then destroyed.

In addition to their local effect, many of the antiseptic and disinfectant drugs have a further poisonous action when they are absorbed and circulate in the blood, and this has led to a further limitation of their use. This general action does not necessarily arise from the qualities which render them antiseptic, and may be avoided by care in the choice of the drug and in its use.

The efficiency of any disinfectant naturally depends on the concentration in which it comes in contact with the microbes and the time during which it remains in contact with them. Thus a solution of mercuric chloride of the strength of 1 in 3000 is much more efficient than one of 1 in 10,000, and after exposure to a solution of five minutes far fewer microbes escape than after exposure for two minutes. Another factor is the temperature at which the microbes are exposed to the disinfectant, for it is found that when the latter is kept at about 30o C. far fewer bacteria escape than when ordinary room temperature prevails. Different species of microbes vary in their resistance, and different cultures of the same microbe and even different individuals of the same culture exhibit marked variations in susceptibility. The effect also often varies inversely with the number of microbes present, because each of these withdraws a certain amount of the disinfectant and thus reduces the general concentration of the solution. And other proteins have the same influence as the microbes themselves, for they offer the disinfectants the same surface for absorption or combine with some of them in the same way as the proteins of the microbes. Thus concentration which is sufficient to sterilize water infected with bacteria, may have little or no effect if applied to a suppurating wound, because the greater part of the disinfectant is taken up or otherwise rendered inactive by the proteins of the secretion; leaving only a low concentration to act on the micro-organisms. Thus Bechold has shown that many substances which are powerful disinfectants in ordinary fluids, lose their activity in protein solutions (commonly called the “protective action of colloids”), owing to their forming combinations with the proteins, so that though they are not dangerous to the host, they are comparatively innocuous to the microbes in the tissues. The inhibiting action of the proteins may also be due partly to their limiting the diffusion of the disinfectant. In fact the antiseptics and disinfectants act on proteins and not especially on the microbes. When the proteins are present in small amount, as in an emulsion of bacteria in water, these disinfectants are active enough, but when the bacteria are distributed among the proteins in an infected wound, the amount of the disinfectant that falls to the share of the bacterial proteins is too small to be effective. For example, a disinfectant which prevented the growth of diphtheria germs in broth when added to the proportion of 1 to 500,000 had no action on the germs in the tissues when it was present in the proportion of 1 to 5000, because it combined with the tissue proteins in preference of those of the bacilli.

Therapeutic Agents Limited in Action

This again indicates the limitation of disinfectant therapeutics, which cannot be overcome as long as the drugs have no elective affinity for the invading organisms but act equally strong on the tissues of the higher animals.

In this connection it might be well to emphasize the point that care should be used in the selection of bactericides in treating abnormal conditions of the mucous membrane of the mouth, the repeated use of germicides of too great potency may defeat the purpose for which they are intended; by lowering the vitality of the surrounding tissue and also inhibiting the action of the phagocytes in their endeavor to combat the infection.

BIBLIOGRAPHY.

A Text-book of Pharmacology and Therapeutics or the Action of Drugs in Health and Disease. By Arthur R. Cushny. Ninth Edition revised by C.W. Edmunds and J.A. Gunn. Copyright Lea and Febiger, 1928. Pages 131 – 134.

A Text-book of Bacteriology by Hans Zinsser. Fifth Edition. Copyright 1922, by D. Appleton and Company. Page 76.

 

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