• most widely used antibiotic
• all penicillins derived from a common nucleus of 6-aminopenicillanic acid which contains a beta-lactam rings joined to a second ring. The beta-lactam ring is needed for the antibiotic properties – the individual properties of each individual penicillin are determined by the additions to the basic beta-lactam ring.
• Weaken and interfere with the synthesis of the peptidoglycan layer of cell wall  cannot withstand osmotic gradient  excessive amounts of water taken up by bacteria  rupture
• Specifically inhibits transpeptidases (enzymes that catalyses the formation of the cross-bridges between the peptidoglycan polymer strands in the cell wall) and activate autolysins (enzymes that cleave bonds in the cell wall)
• need to bind to penicillin binding proteins (PBP’s) – 8 different types of PBP’s located on outside of cytoplasmic membrane
• have no direct effect on host (almost makes them an ideal antibiotic) – mammalian cells do not have a cell wall
• active against both gram positive and some gram negative (cell wall structure of gram negative has an additional out layer that is not easily penetrated by penicillins to reach PBP’s)
• development of resistance due to beta-lactamase production  cleave the beta-lactam ring  inactivates penicillin (can be inhibited by clavulanic acid and sulbactam); the beta-lactases that act against penicillin are called penicillinases.
• excreted by tubular secretion (can be blocked by probenecid  prolonged effect)
• main adverse reaction is allergy – occurs in 1%-10%
• contraindications – allergy

Penicillin G/Benzylpenicillin/”Penicillin”:
• first penicillin available
• effective against most gram-positive (except penicillinase producing staphylococci), gram negative cocci, anaerobic bacteria. Gram-negative bacilli are resistant.
• indicated as first choice for many infection

Phenoxymethylpenicillin (penicillin 5):
• ineffective against organisms that produce pencillinase

• eg Alphacin®, Alphamox®, Amohexal®, Austrapen®, Bgrmin®, Cilamox®, Moxacin®

Amoxycillin with clavulanic acid*
• clavulanic acid inhibits beta-lactamase  increases spectrum of amoxycillin
• eg Augmentin® (amoxycillin trihydrate and potassium clavulanate), Ausclav®, Clamoxyl®,

• eg Diclocil®, Dicloxsig®, Distaph®

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