antibiotic drugs A very extensive range of drugs able to kill or prevent reproduction of bacteria in the body without killing the patient. Antibiotics were originally derived from cultures of living organisms, such as fungi or bacteria, but, today, many are chemically synthesized. The antibiotics have enormously extended the scope and effectiveness of medical therapy against bacterial infection, but have not succeeded in eliminating any bacterial diseases. The extensive, and not always judicious, use of antibiotics has led to widespread evolutionary changes in bacteria in response to their new environment, manifested by the acquisition, by natural selection, of resistance to these drugs. This forces researchers to produce ever new and more effective antibiotics. The antibiotics include such classes as the aminoglycosides, amphenicols, ansamycins, lincosamides, macrolides, polypeptides, tetracyclines and beta-lactams. The beta-lactams include groups such as carbapenems, cephalosporins, cephamycins, monobactams, oxacephems and penicillins.