penicillin (pen·i·cil·lin) (pen²[ibreve]-sil¢in)  any of a large group of natural or semisynthetic antibacterial antibiotics derived directly or indirectly from strains of fungi of the genus Penicillium and other soil-inhabiting fungi grown on special culture media, which exert a bacteriocidal as well as a bacteriostatic effect on susceptible bacteria by interfering with the final stages of the synthesis of peptidoglycan, a substance in the bacterial cell wall. The penicillins, despite their relatively low toxicity for the host, are active against many bacteria, especially gram-positive pathogens (streptococci, staphylococci, pneumococci); clostridia; some gram-negative forms (gonococci, meningococci); some spirochetes (Treponema pallidum and T. pertenue); and some fungi. Certain strains of some target species, e.g., staphylococci, secrete the enzyme penicillinase, which inactivates penicillin and confers resistance to the antibiotic.

aluminum penicillin,  the aluminum salt of penicillin prepared from extracts of cultures of Penicillium notatum or P. chrysogenum.

benzathine penicillin,  p. G benzathine.

benzyl penicillin potassium,  p. G. potassium.

benzyl penicillin sodium,  p. G sodium.

clemizole penicillin,  the clemizole salt of penicillin G, the combination of which produces a repository form of penicillin G with antihistaminic properties.

dimethoxyphenyl penicillin sodium,  methicillin sodium.

penicillin G,  the most widely used form and the first of the penicillins developed for medicinal use. It is used in the form of the benzathine, potassium, procaine, and sodium salts, principally in the treatment of infections due to penicillin-susceptible gram-positive bacteria, gram-negative cocci, Treponema pallidum, and Actinomyces israelii. Called also benzylpenicillin.

penicillin G benzathine, [USP]  a salt having a long-sustained action, obtained by combining penicillin G with N,N¢-bis(phenylmethyl)-1,2-ethanediamine (2:1); administered orally and intramuscularly.

penicillin G potassium,  a salt of penicillin G, administered orally and by intravenous injection or infusion.

penicillin G procaine,  a salt having a long-sustained action, obtained by combining penicillin G with procaine (1:1); administered intramuscularly.

penicillin G sodium, [USP]  a salt having a potency of 1500–1750 U per mg; administered intramuscularly and intravenously.

isoxazolyl penicillin,  a group of semisynthetic penicillins, including oxacillin, cloxacillin, and dicloxacillin, which combine resistance to penicillinase with acid stability and activity against gram- positive bacteria.

penicillin N,  adicillin.

penicillin O,  a penicillin produced biosynthetically by adding a precursor to the culture medium; penicillin O and its potassium and sodium salts have actions similar to those of penicillin G and are said to be hypoallergenic.

penicillin O potassium,  see p. O.

penicillin O sodium,  see p. O.

phenoxymethyl penicillin,  p. V.

potassium phenoxymethyl penicillin,  p. V potassium.

penicillin V, [USP]  a semisynthetic oral penicillin prepared from cultures of the mold Penicillium in the presence of 2-phenoxyethanol with an autolysate of yeast as the source of nitrogen. It is a broad-spectrum antibiotic having pharmacologic and toxic properties similar to those of other penicillins, and is less potent than penicillin G. Called also phenoxymethyl p.

penicillin V benzathine, [USP]  the benzathine salt of penicillin V, administered orally.

penicillin V potassium, [USP]  the potassium salt of penicillin V, administered orally.