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. By the late 20th century, concerns arose that the widespread use of tetracyclines was contributing to an increase in the number of tetracycline-resistant organisms, which in turn had rendered certain infections more resilient to treatment. The use of tetracyclines in livestock feed to promote growth was also called into question.