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

The production and use of theantibioticpenicillin in the early 1940s became the basis for the era of modernantimicrobial therapy.Streptomycin was discovered in 1944, and since then many other antibiotics and other types of antimicrobials have been found and put into use. A major discovery following the introduction of these agents intomedicine was the finding that their basic structure could be modified chemically to improve their characteristics. Thus,antimicrobial agents that are used in thetreatment of disease includesynthetic chemicals as well as chemical substances or metabolic products made by microorganisms and chemical substances derived fromplants.

The termantibiotic is popularly used to refer todrugs that combat microbes, but, strictly speaking, it refers only to drugs that kill orinhibitbacteria.

Antiseptics andgermicides

The termantiseptic 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 combatbacterial infections of superficial tissues and to sterilize instruments and infected material. A distinction must be made between antiseptics andchemotherapeutic agents, such as antibiotics andsulfonamides, 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. (Seeantibiotic.)

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

Theefficiency 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 asphenol lose their activity sharply beyond a certain dilution, whereasmercurial 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 andchlorine salts) act quickly, whereasmercurials, compounds of heavy metals, anddyes act slowly. Most antiseptics act more quickly under increased temperatures; the activity ofcoal 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.

Alcohols are among the most widely used antiseptics, especially ethyl andisopropyl alcohol, which are commonly used in a 70 percent concentration with water. They are also widely used in combination with other antiseptic agents. Thephenols contain a large number of common antiseptics and disinfectants, among them phenol (carbolic acid) andcreosote, while such bisphenols ashexyl resorcinol andhexachlorophene are widely used as antiseptic agents in soaps.Chlorine andiodine 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 itsderivatives, the hypochlorite solutions (e.g.,Dakin’s solution) are used in surgical practice. Iodine is an effectivedisinfectant 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. Mostacids andalkalis either are too caustic to tissues or are relatively inefficient bactericides.

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Classification and survey of antiseptics and germicides

Antiseptics and germicides are classified and described in the table.