Any of a class of broad-spectrum antibiotics with a common basic structure, including doxycycline. They may be isolated directly from several species of actinomycetes of the genus Streptomyces or modified from the compounds isolated. They are the drugs of choice in the treatment of cholera, rickettsial infections (see rickettsia), psittacosis, brucellosis, and tularemia; tetracyclines are also used in the treatment of acne. Overuse of these and other antibiotics has led to drug resistance in microorganisms