benzylpennicillin

Benzylpenicillin, commonly known as penicillin G, is a naturally occurring β‑lactam antibiotic first isolated from the mold Penicillium notatum in 1940 by Alexander Fleming. It is the prototype of the penicillin class and remains a crucial first‑line treatment for a variety of bacterial infections. The drug is effective primarily against Gram‑positive cocci such as Staphylococcus aureus, Streptococcus pyogenes, and Streptococcus pneumoniae, and it is also active against some Gram‑negative organisms, including Neisseria meningitidis.

Its bactericidal activity is mediated by binding to penicillin‑binding proteins (PBPs) located in the bacterial cell‑wall synthesis machinery. By covalently attaching to the active site of PBPs, benzylpenicillin inhibits the transpeptidation step of peptidoglycan cross‑linking, causing cell lysis. In addition to its therapeutic uses, the drug can be employed as a laboratory reagent for the detection of bacterial β‑lactamases.

Commercial preparations of benzylpenicillin may be administered parenterally as sodium or potassium salts, or taken orally in the form of prodrug penicilloyl potassium or penicilloyl sodium salts that are converted to the active drug in situ. The most commonly prescribed routes of administration are intravenous, intramuscular, and oral.

Side effects include allergic reactions ranging from urticaria to anaphylaxis, gastrointestinal discomfort, and, rarely, myoclonus. Resistance to benzylpenicillin arises in part through the production of β‑lactamases, altered PBPs, or reduced membrane permeability. Despite these limitations, benzylpenicillin remains an essential antimicrobial agent for many community and hospital infections.