chloramphenicol (klōrˌămfĕnˈəkŏlˌ) [key], antibiotic effective against a wide range of gram-negative and gram-positive bacteria (see Gram's stain). It was originally isolated from a species of Streptomyces bacteria. Chloramphenicol's antibiotic activity results from its interference with protein synthesis in invading microbes. However, it is a very toxic substance, its most serious and potentially lethal effect being depression of red blood cell production in bone marrow; cases of leukemia were also attributed to early use of chloramphenicol. Because of its toxicity, chloramphenicol is rarely prescribed for infections that can be treated by other antibiotics. It is used as an alternative therapy to treat typhoid fever, some forms of meningitis, and rickettsial infections such as Rocky Mountain spotted fever and typhus. Chloramphenicol is commonly used in biological research to study protein synthesis. Chloromycetin is a trade name for chloramphenicol.