PENICILLINS

· Beta-lactam antibiotic

· From Penicillium sp

· Interferes with bacterial cell-wall synthesis in growing & dividing bacteria thereby resulting in bacteria which have defective cell walls

· Therefore, penicillins exert their action most effectively during the growing phase of bacteria

· All penicillins consist of thiazolidine ring [A] attached to a beta-lactam ring [B]

· If the beta-lactam ring is enzymatically cleaved by bacterial betalactamases (penicillinases), the resulting product is devoid of antibacterial activity

Natural Penicillins [Benzyl penicillin (penicillin G)]

· Produced biosynthetically by fermentation of Penicillin

· Effective against sensitive gram-positive bacteria

· Least toxic of the antibiotics (eg nephrotoxicity) however hypersensitivity reactions commonly occur

Semi-synthetic Penicillins [Methicillin;Flucloxacillin]

· Prepared by adding different side chains to the penicillin nucleus (A-B)

· Penicillinase-resistant - effective against penicillin resistant bacteria

· Used in severe infections due to penicillinase-producing staphylococci eg septicaemia, meningitis, pneumonia

Broad spectrum semi-synthetic Penicillins [Ampicillin; Amoxycillin; Carbenicillin; Ticarcillin; Piperacillin]

· Are semisynthetic penicillins with a wider antibacterial spectrum

· Effective against both gram positive & gram negative bacteria

· Ampicillin & Amoxycillin are not effective against penicillinase producing bacteria

· Carbenicillin and Ticarcillin are uniquely effective against pseudomonas

· Piperacillin has a wider spectrum of activity (4th generation penicillin)