penicillin /pen′isil″in/ [L, penicillus, paintbrush] , any one of a group of antibiotics derived from cultures of species of the fungus Penicillium or produced semisynthetically. Various penicillins administered orally or parenterally for the treatment of bacterial infections exert their antimicrobial action by inhibiting the biosynthesis of cell-wall mucopeptides during active multiplication of the organisms. Penicillin G is a widely-used therapeutic agent for meningococcal, pneumococcal and streptococcal infections; syphilis; and other diseases. It is rapidly absorbed when injected intramuscularly or subcutaneously, but it is inactivated by gastric acid and hydrolyzed by penicillinase produced by most strains of Staphylococcus aureus. Penicillin V is also active against gram-positive cocci, with the exception of penicillinase-producing staphylococci, and, because it is resistant to gastric acid, it is effective when administered orally. Penicillins resistant to the action of the enzyme penicillinase (beta-lactamase) are cloxacillin, dicloxacillin, methicillin, nafcillin, and oxacillin. Ampicillin and amoxicillin are broad-spectrum aminopenicillins active against gram-negative organisms, including Escherichia coli, Haemophilus influenzae, Neisseria gonorrhoeae, Salmonella, Shigella, and Proteus mirabilis. Extended-spectrum penicillins include carbenicillin, piperacillin, and ticarcillin. These drugs are effective against the same bacteria killed by the aminopenicillins and are also effective against a number of additional bacteria, including species of Pseudomonas, Enterobacter, Klebsiella, Proteus, and Bacteroides. Hypersensitivity reactions are common in patients receiving penicillin and may appear in the absence of prior exposure to the drug, presumably because of unrecognized exposure to a food or other substance containing traces of the antibiotic. The most common hypersensitivity reactions are rash, fever, and bronchospasm, followed in frequency by vasculitis, serum sickness, and exfoliative dermatitis. In some patients severe erythema multiforme accompanied by headache, fever, arthralgia, and conjunctivitis (Stevens-Johnson syndrome) develop. The most frequent cause of anaphylactic shock is an injection of penicillin.
