Penicillin refers to a class of antibiotic drugs that have a common -lactam core structure. Characteristics of penicillins, especially bioavailability, can vary substantially depending upon the R- group appended onto the core structure. Some penicillin drugs are actually prodrugs that only become active after some metabolitic transformation within a patient. Penicillin drug may cause severe, even fatal, allergic responses in some patients. Some bacteria have developed resistance to penicillin through the acquisition of -lactamases, enzymes which degrade the -lactam structures of the penicillin drugs.
Although the discovery of penicillin is often attributed to Alexander Fleming, organisms which produce them have been used since ancient times to treat infections. Natural and semisythetic penicillins are produced by fungi: the original strain producing penicillin G (benzylpenicillin) was Penicillium notatum, although the industry changed to a higher-yielding P. chrysogenum.
Semisynthetic penicillins have precursor chemicals added to the culture media, which are incorporated into the antibiotic molecule. The modifications variously improve coverage of Gram-negative organisms, beta-lactamase resistance, or both. Another approach to resisting penicillinase is to co-administer a competive inhibitor of the destructive enzyme, such as clavulanic acid with amoxicillin.
For more information about specific penicillin drugs, see these pages: