Antibiotics are chemical substances which are produced by microorganisms and have the capacity to inhibit the growth of and even to destroy bacteria and other microorganisms. They are characterized by certain distinct physical, chemical, and biological properties which make them ideal potential chemotherapeutic agents for the treatment of infection (Louis Weinstein). These properties can be described briefly.
1. Antibiotics are selective in their effect on different microorganisms, being specific in their action not only against genera and species but even against strains and individual cells. Some of these agents act mainly on gram-positive bacteria, while others inhibit only gram-negative ones. Others
affect alike various types of bacteria regardless of their tinctorial properties. Some have no effect upon fungi, while others, although too toxic to be clinically useful at present, are actively fungistatic. Certain antibiotic agents are effective against rickettsiae and a few of the larger viruses. The variations in the action of these drugs upon different bacteria and other parasites are both qualitative and quantitative; this has led to the development of the concept of an antibiotic spectrum, which records the selective action of a given antibiotic upon a number of representative bacteria and other microorganism.2. The antibiotic agents represent a large number of chemical compounds, ranging from simple substances containing only carbon, hydrogen, and oxygen, to more complex forms which contain nitrogen, sulfur, and even chlorine. They vary greatly in chemical structure.
3. Certain microorganisms are
capable of producing more than one antibiotic. Streptomyces griseus, for example, produces streptomycin and mannosidostreptomycin, the antifungous agent actidione, and the antitrichomonas substance, streptocin.4. Some antibiotics are produced by several different organisms. Thus various penicillins are formed by different strains of Penicillium.
5. Salts and serum proteins, among other factors, may reduce the effectiveness of an antibiotic by neutralizing, adsorbing, or inactivating it.
6. Some antibiotics, penicillin for example, are rapidly destroyed by various bacteria, whereas others, such as streptomycin, are highly resistant to microbial action.
7. The mode of action of antibiotics upon organisms varies. Some interfere with growth and cell division. Others affect microbial respiration or the utilization of essential metabolites.
8. Antibiotics vary greatly in their toxicity for animals and man.
9. Most antibacterial agents are potentially capable of eliciting various types of hypersensitivity reactions in patients to whom they are administered.
10. Bacteria sensitive to an antibiotic may gradually develop resistance after contact for varying periods. Different antibiotics vary greatly in this respect. With streptomycin, rapid development of insensitivity is common; penicillin allows only gradual development of resistance of very few sensitive bacteria.
11. The application of antibiotics in the chemotherapy of infections may produce pronounced alterations in the normal bacterial flora of the body, especially in the mouth, pharynx, and gastrointestinal tract.