tetracycline, any of a group of broad-spectrum antibiotic compounds that have a common basic structure and are either isolated directly from several species of Streptomyces bacteria or produced semisynthetically from those isolated compounds.
Tetracyclines act by interfering with the ability of a bacterium to produce certain vital proteins; thus, they are inhibitors of growth (bacteriostatic) rather than killers of the infectious agent (bacteriocidal) and are effective only against multiplying microorganisms. Tetracyclines are used against a variety of infectious diseases, including cholera, rickettsial infections, trachoma (a chronic infection of the eye), psittacosis (a disease transmitted by certain birds), brucellosis, and tularemia. Tetracyclines have also been used for the treatment of acne.
While all tetracyclines have a common structure, they differ from each other by the presence or absence of chloride, methyl, and hydroxyl groups. Although these modifications do not change their broad-spectrum antibacterial activity, they do affect pharmacological properties such as half-life and binding to proteins in serum. The tetracyclines all have the same antibacterial spectrum, although there are some differences in sensitivity of the bacteria to the various types of tetracyclines. They inhibit protein synthesis in both bacterial and human cells. Bacteria have a system that allows tetracyclines to be transported into the cell, whereas human cells do not; human cells therefore are spared the effects of tetracycline on protein synthesis.
All tetracyclines are absorbed from the gastrointestinal tract after oral administration, and most can be given intravenously or intramuscularly. Because calcium, magnesium, aluminum, and iron form insoluble products with most tetracyclines, they cannot be given simultaneously with substances containing these minerals (e.g., milk). Complexes between tetracyclines and calcium can cause staining of teeth and retardation of bone growth in young children or in newborns if tetracyclines are taken after the fourth month of pregnancy. Tetracycline can also cause photosensitivity in patients exposed to sunlight.
Because not all of the tetracycline administered orally is absorbed from the gastrointestinal tract, the bacterial population of the intestine can become resistant to tetracyclines, resulting in overgrowth (suprainfection) of resistant organisms. The widespread use of tetracyclines is thought to have contributed to an increase in the number of tetracycline-resistant organisms, in turn rendering certain infections more resilient to treatment. The use of tetracyclines in livestock feed to promote growth has also been called into question.