1. Benzylpenicillin, Procaine penicillin, phenoxy- methyl penicillin, phenethicillin
2. Penicillinase-resistant penicillins (Cloxacillin, methicillin, flucloxacillin)
3. Broad spectrum penicillins (Ampicillin, Amoxycillin, Carbenicillin)
Penicillin acts by interfering with cell wall mucopeptide synthesis so that organisms explode from internal pressure. Penicillin is thus bactericidal and only effective against multiplying organisms since resting organisms are not making new cell walls.
Benzylpenicillin is not given orally because it is destroyed by acid in the stomach and poorly absorbed from intestines. It is active against Gram-positive bacilli, both Gram positive and Gram negative cocci and spirochaetes.
Penicillinase-resistant penicillins are invaluable in treating staphylococcal infections, which are penicillinase (alactamase) producing organisms such as ''staphylococcus aureus''. Amoxycillin, ampicillin and other semi-synthetic Penicillins have much broader spectrum, which are bactericidal to Gram-negative bacilli including 'Salmonellae', 'Shigellae', E-coli, H. influenzae and some Proteus strains. A combination of Cloxacillin and ampicillin is very effective in treatment of neonatal infection which may include resistant staphylococci. Amoxycillin is pharmacokinetically superior to ampicillin i.e. smaller doses give effective plasma concentrations.
