Antibiotics are chemical substances produced by various species of microorganisms and other living systems that are capable in small concentrations of inhibiting the growth of or killing bacteria and other microorganisms. These organisms can be bacteria, viruses, fungi, or animals called protozoa. A particular group of these agents is made up of drugs called antibiotics, from the Greek word anti ("against") and bios ("life"). Some antibiotics are produced from living organisms such as bacteria, fungi, and molds. Others are wholly or in part synthetic - that is, produced artificially.

Penicillin is perhaps the best known antibiotic. Its discovery and later development is among mankind's greatest achievements. Antibiotics have enabled the medical profession to treat effectively many infectious diseases, including some that were once life-threatening.

Antibiotics can be bacteriostatic (bacteria stopped from multiplying) or bactericidal (bacteria killed). To perform either of these functions, antibiotics must

Current work in antibiotics is largely in the area of viruses. Although some antivirals are available, most have toxic effects so severe that they can be used only in life-threatening diseases where the negative effects are the lesser danger. Preliminary studies, however, are reporting success in the development of safer antiviral drugs, and their use should be possible within the near future.

When one is exposed continually to an antibiotic for an illness of long duration (such as rheumatic fever), the targeted bacteria may develop its own defense against the drug. An enzyme that can destroy the drug may be produced by the bacteria, or the cell wall can become resistant to being broken by the action of the antibiotic. When this happens, and it does most frequently in response to long or frequent treatment with penicillin or streptomycin, the patient is said to be "fast" against the drug. For example, one may be penicillin-fast, meaning penicillin is no longer able to help fight the infection and another type of antibiotic must be given.

Polypeptides. The class of antibiotics called polypeptides is quite toxic (poisonous) and is used mostly on the surface of the skin (topically). Bacitracin is in this category.

Cephalosporins. Similar to the penicillins, cephalosporins are often given when a sensitivity (allergic reaction) to the former is known or suspected in a patient. Cefotaxime sodium is a kind of cephalosporin that is very effective in combating deep infections such as those that occur in bones and those resulting from surgery.

There are dozens of antibiotics. The following are in common use:

An antibiotic acts by limiting or stopping (and therefore killing) the growth of a specific microorganism. It probably accomplishes this by interfering with the wall of the bacteria cell at which it is targeted while at the same time having little effect on the body's normal cells.

Topical. Topical application means "to a local area" such as on the skin, in the eyes, or on the mucous membrane. Antibiotics for topical use are available in the form of powders, ointments, or creams.

Macrolides. Macrolides are often used in patients who appear to be sensitive to penicillin. Erythromycin is the best known medicine in this group.

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