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Section IVOther important drugsSpecific penicillins
Narrow spectrum penicillin
Benzylpenicillin(penicillin G) is inactivated by gastric acid and must be given par-
enterally. It is active against a wide variety of gram-positive pathogens, gram-negative
cocci, and occasional gram-negative bacilli. Streptococci and Neisseria are extremely
sensitive, whilst someProteus mirabilis(P.mirabilis), Salmonella andEnterococcus
faecalis(E.faecalis)aremildly so.H. influenzae,staphylococci, and Pseudomonas are
resistant. Some anaerobic gram-positive cocci are sensitive butBacteroides fragilis
(B. fragilis)isresistant.Narrow spectrum penicillins resistant to staphylococcalβ-lactamase
Most staphylococci are resistant to benzylpenicillin as a result ofβ-lactamase
production. However, the semi-synthetic penicillinflucloxacillinis moderately
resistant and provides effective therapy in this setting. It is well absorbed from
the gut but should be given intravenously for serious infections. It is less active
against gram-positive cocci than benzylpenicillin but is effective againstβ-lactamase
positive staphylococci. Metabolism produces both active and inactive metabo-
lites. It may cause cholestatic jaundice several weeks after the end of a course of
treatment.Extended spectrum penicillins
Ampicillinis effective against the same range of organisms as benzylpenicillin
(although slightly less active) as well as activity against someH. influenzae,
Salmonella,Escherichia coli(Esch. coli), andE.faecalis.Amoxycillinhas an iden-
tical spectrum to ampicillin but has better bioavailability and is bactericidal to sus-
ceptible gram-negative organisms at a lower concentration. Both are inactivated by
β-lactamase.Clavulanic acidirreversibly inhibits a large range ofβ-lactamases and
when combined with amoxycillin as Co-Amoxiclav reduces the minimum inhibitory
concentration (MIC) againstH. influenzae, Branhamella catarrhalis(B. catarrhalis),
Esch. coli, B. fragilis,Klebsiella andStaphylococcus aureus(Staph. aureus)eight to
sixty-four-fold. Clavulanic acid is weakly antimicrobial in its own right and may also
be responsible for the rare occurrence of cholestatic jaundice several weeks after
treatment has been stopped.Anti-pseudomonal penicillins
The carboxypenicillin ticarcillin and the ureidopenicillins azlocillin and
piperacillinhave a broad spectrum (albeit with a lower activity than benzylpeni-
cillin) and are particularly indicated for use against Pseudomonas, Citrobacter and
Serratia. They areβ-lactamase sensitive so ticarcillin is presented with clavulanic
acid and piperacillin is presented with tazobactam (aβ-lactamase inhibitor with no