What is an antibiotic made of?

Antibiotics were once naturally occurring substances produced by an organism to inhibit the growth of another organism.

But now most antibiotics are semisynthetic-modified chemically from original compounds found in nature, as is the case with beta-lactams (which include the penicillins, produced by fungi in the genus Penicillium, the cephalosporins, and the carbapenems).

Some antibiotics are still produced and isolated from living organisms, such as the aminoglycosides, and others have been created through purely synthetic means: the sulfonamides, the quinolones, and the oxazolidinones.