antimicrobial agent

▪ pharmacology

Introduction

      any of a large variety of chemical compounds and physical agents that are used to destroy microorganisms or to prevent their development.

Antiseptics (antiseptic) and germicides.

      The term antiseptic refers to agents applied to the living tissues of humans, other animals, and plants in order to destroy (bactericidal) or inhibit the growth (bacteriostatic) of infectious microorganisms. Antiseptics are used in medical practice to prevent or combat bacterial (bacteria) infections of superficial tissues and to sterilize instruments and infected material. A distinction must be made between antiseptics and chemotherapeutic agents, such as antibiotics and sulfonamides, which are administered by mouth or by injection for the treatment of internal or generalized infections but may also be applied locally in the treatment or prevention of superficial infections. (See antibiotic.)

      Many chemical compounds can kill bacteria, but many of them also exhibit properties that limit or prohibit their use. Most antiseptics are general protoplasmic poisons and if used in sufficient concentration are harmful to the body's cells and tissues as well as to bacteria. Thus an antiseptic is most valuable in the disinfection of contaminated wounds or skin surfaces when there is a wide margin between its bactericidal and toxic concentrations. When, however, an antiseptic is to be used to disinfect contaminated instruments or other inanimate objects, its toxic properties are not important, and many compounds (called disinfectants (disinfectant)) may be used that cannot be applied to living tissues. The term disinfectant thus refers to substances that are used to destroy microorganisms on inanimate surfaces—e.g., surgical instruments, floors, and walls (see disinfectant). Antiseptics, disinfectants, and antibiotics are all germicides—i.e., they are all substances that kill microorganisms.

      The efficiency of an antiseptic must be measured in relation to three main factors: concentration, time, and temperature. It is desirable to know the minimum concentration at which an antiseptic will be effective. Some antiseptics such as phenol lose their activity sharply beyond a certain dilution, whereas mercurial preparations still inhibit bacterial growth at very high dilutions. The time that an antiseptic takes to act depends to some extent on its concentration, but the speed at which different antiseptics kill bacteria varies considerably; thus the halogens (e.g., iodine and chlorine salts) act quickly, while mercurials, compounds of heavy metals, and dyes act slowly. Most antiseptics act more quickly under increased temperatures; the activity of coal-tar derivatives, for instance, is doubled by a rise in temperature from that of a cool room to body heat. Many antiseptics destroy certain types of microorganisms and not others. Many others will kill bacteria but not their spores, which are walled, usually dormant, reproductive bodies.

       Classification and survey of antiseptics and germicidesThe Table (Classification and survey of antiseptics and germicides) gives the major families of antiseptics. Alcohols (alcohol) are among the most widely used antiseptics, especially ethyl and isopropyl alcohol, which are commonly used in a 70-percent concentration with water. They are also widely used in combination with other antiseptic agents. The phenols (phenol) contain a large number of common antiseptics and disinfectants, among them phenol (carbolic acid) and creosote, while such bisphenols as hexyl resorcinol and hexachlorophene are widely used as antiseptic agents in soaps. chlorine and iodine are both extremely effective agents and can be used in high dilution. Chlorine is widely used in the disinfection of drinking-water supplies, and among its derivatives, the hypochlorite solutions (e.g., Dakin's solution) are used in surgical practice. Iodine is an effective disinfectant of wounds, particularly when used in an alcohol solution. The salts of most metals are generally too toxic to use on living tissues, but complex organic mercury salts (e.g., mercurochrome, merthiolate) in alcohol solution are highly bacteriostatic and make useful wound disinfectants. The quaternary ammonium compounds are more widely used as disinfectants than as antiseptics. Certain acridine dyes are used as antiseptics, as are some aromatic, or essential, oils. Most acids and alkalis are either too caustic to tissues or are relatively inefficient bactericides.

Sterilization.

      Sterilization, which is any process, physical or chemical, that destroys all forms of life, is used especially to destroy microorganisms, spores, and viruses. Precisely defined, sterilization is the complete destruction of all microorganisms by a suitable chemical agent or by heat, either wet steam under pressure at 120° C (250° F) or more for at least 15 minutes, or dry heat at 160° to 180° C (320° to 360° F) for three hours.

Sanitization.

      A sanitizer is an agent, usually chemical in nature, that is used to reduce the number of microorganisms to a level that has been officially approved as safe. Sanitizers are commonly used to control bacterial levels in equipment and utensils found in dairies, other food-processing plants, eating and drinking establishments, and other places in which no specific pathogenic microorganisms are known to be present and destruction of all microorganisms may not be necessary.

Other antimicrobials.

      Preservatives (preservative), usually chemical agents, are added to certain foods and medicines to prevent the growth of microorganisms that may cause spoilage or disease. Prophylactics also are agents used to prevent infections and diseases. Vaccination is the administration of harmless amounts of disease-causing microorganisms into animals, including humans, to prevent diseases. (See vaccine.) Sterile filtration usually removes large microorganisms (e.g., bacteria, fungi, and their spores) from heat-sensitive solutions, but this physical method does not effectively remove small infectious microorganisms (e.g., filterable viruses and rickettsias).

Modes of action.

      Some antiseptics, such as alcohols and quaternary ammonium compounds, act directly on microbial cells to dissolve them. Others may penetrate the cells and cause the release of amino acids, nuclear material, and other important chemical constituents. Some compounds penetrate microbial cell walls and inactivate essential membrane transport systems, so that the cells can no longer obtain the nutrients necessary for them to survive and to reproduce. Others coagulate certain vital materials in cells, thereby destroying the microorganisms. A few agents disrupt the metabolism of the cells, so that they can no longer assimilate nutrients; as a result, the cells starve and die.

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