Microorganisms can grow on and within other organisms which leads to the development of disease, disability and death of an organism. Thus the control or destruction of microorganisms residing within the bodies of humans and other animals is of great importance. Modern medicine is dependent on chemotherapeutic agents, chemical agents that are used to treat diseases. Most of them are antibiotics, microbial products or their derivatives that can kill susceptible microbes or inhibit their growth.

The development of chemotherapy began with the work of German physician Paul Ehrlich, who reasoned that a chemical with selective toxicity that would kill pathogens and not human cells might be effective in treating disease. In 1904 he found that the dye trypan red was active against the trypanosome that causes African sleeping sickness and could be used therapeutically. Arsphenamine was active against syphilis. Dogmagk discovered that Protonoid red, a dye for staining leather, was nontoxic for animals and completely protected mice against pathogenic streptococci and staphylococci. Dogmagk has actually discovered sulphonamides or sulfa drugs and for this discovery he received Nobel Prize in 1939. The first antibiotic to be used therapeutically was penicillin. Stephan Fleming was the first to discover penicillin from the fungus Penicillium notatum. Laterother scientists, Flare and Chain carried out the work further and the three scientists were awarded with Nobel Prize in 1945 for the discovery and production of penicillin. After the discovery of Penicillin, scientists became interested in other compounds produced by microbes which could be used as antibiotics. Selman Waksman announced in 1944 that he and his associates has found a new antibiotic, streptomycin, produced by the actinomycete Streptomyces griseus . He received the Nobel Prize in 1952 and his success led to a worldwide search for other antibiotic-producing microbes. Microorganisms producing chloramphenicol, neomycin, Terramycin and tetracycline were isolated by 1953. This led to the discovery of powerful drugs and has transformed modern medicine and greatly alleviated human suffering.

General characteristics and mechanisms of action of antimicrobial agents:

A successful chemotherapeutic agent must have selective toxicity. The degree of selective toxicity may be expressed in terms of 1) the therapeutic dose, the drug level required for treatment of a particular infection and 2) the toxic dose, the drug level at which the agent becomes too toxic for the host. The therapeutic index is the ratio of the toxic dose to the therapeutic dose. The larger the TI, the better the chemotherapeutic agent. A drug that disrupts a microbial function not found in eukaryotic animal cells has a greater selective toxicity and a higher therapeutic index. Example, therapeutic index of penicillin is high as the action of penicillin is that it inhibits the peptidoglycan synthesis of bacterial cell walls, and as eukaryotic cells do not contain cell walls, the effect is minimum. The drug which has a low therapeutic index can cause undesirable effects on the host, called as side effects as the drug may inhibit the same process in host cells or damages the host in other ways. Hence, chemotherapeutic agents must be administered with great care.