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The Collaborative International Dictionary of English v.0.48 (gcide)

penicillin \pen`i*cil"lin\ (p[e^]n`[i^]*s[i^]l"l[i^]n), n. [From
Penicillium, the fungus genus from which it was first
isolated.] (Chem.)
Any of a variety of substances having a structure containing
a beta-lactam ring fused to a thiirane ring, to which a
carboxyl group is attached, but most commonly interpreted as
{benzyl penicillin}. They are notable as powerful
antibacterial agents of relatively low toxicity which have
found extensive use in medicine for treating bacterial
infections. They are categorized as one of the classes of
beta-lactam antibiotic. They are produced naturally by some
fungi and bacteria, and industrial production processes
almost invariably start from some form of the penicillin
nucleus produced by fermentation of microorganisms. The
fermentation products are then chemically modified to produce
derivatives of enhanced potency, safety, or antibacterial
spectrum. The first penicillin to see extensive use
clinically (during World War II) was {penicillin G}, also
called {benzypenicillin}, and commonly simply "penicillin".
[PJC]

Note: [hand]The characteristic portion of the structure
characterizing a penicillin is the bicyclic structure
3,3-Dimethyl-7-oxo-4-thia-1-azabicylo[3.2.0]heptane-2-carboxylic
acid. The different penicillins have varying acyl
groups forming an amide bond with a nitrogen attached
to the 6-carbon of this nucleus. This distinguishes it
from the other classes of beta-lactam antibiotic, the
{cephalosporins}, {thienamycins} and {sulfazecin}.
[PJC]


Lactam \Lac"tam\, n. [Lactone amido.] (Chem.)
One of a series of anhydrides of an amido type, analogous to
the lactones, as oxindol; a cyclic amide.
[1913 Webster]

{beta-lactam} or {[beta]-lactam},
(a) a lactam in which the amide bond is contained within a
four-membered ring, which includes the amide nitrogen and
the carbonyl carbon.
(b) an antibiotic containing a beta-lactam, such as a
{penicillin}, {cephalosporin}, or {carbapenem}; also
called a {beta-lactam antibiotic}. [informal, laboratory
slang]
[PJC]