Amoxicillin is a moderate-spectrum antibiotic active against a wide range of Gram-positive, and a limited range of Gram-negative organisms. Some examples of susceptible and resistant organisms, from the Amoxil Approved Product Information (GSK, 2003), are listed below.
Susceptible Gram-positive organisms
— Streptococcus spp.
— penicillin-susceptible Streptococcus pneumoniae
— non β-lactamase-producing Staphylococcus spp.
— Enterococcus faecalis.
Susceptible Gram-negative organisms
Non-β-lactamase producing strains of the following bacteria:
— Haemophilus influenzae
— Neisseria gonorrhoeae
— Neisseria meningitidis
— Escherichia coli
— Proteus mirabilis
— Salmonella spp.
Penicillinase-producing organisms, particularly penicillinase-producing Staphylococcus spp. Penicillinase-producing N. gonorrhoeae and H. influenzae are also resistant.
All strains of Pseudomonas spp., Klebsiella spp., Enterobacter spp., indole-positive Proteus spp., Serratia marcescens, and Citrobacter spp. are resistant.
The incidence of β-lactamase-producing resistant organisms, including E. coli, appears to be increasing. Doctors may opt to prescribe co-amoxiclav, amoxicillin combined with β-lactamase inhibitor potassium clavulanate, instead of just amoxicillin to increase the efficacy.
Doubling the routinely given concentration (in pediatrics) of amoxicillin has been shown to eradicate intermediately resistant Streptococcus pneumoniae in selected infections.
Amoxicillin in trihydrate form is available as capsules, chewable and dispersable tablets plus syrup and paediatric suspension for oral use, and as the sodium salt for intravenous administration. It is one of the most common antibiotics issued to children, and the sweet-tasting liquid forms are helpful where the patient might find it difficult to take tablets or capsules. It has 3 ionizable groups.
Amoxicillin and clavulanic acid
To overcome the issue of β-lactamase production by resistant organisms, amoxicillin (in either trihydrate or sodium salt forms) may be combined with clavulanic acid, typically as the potassium salt. This combination has activity against a very broad array of Gram-positive, Gram-negative, and anaerobic organisms. It is not active against MRSA, P. aeruginosa, or C. difficile. It is available in oral preparations worldwide and also in the intravenous preparation in some countries. The British Approved Name for this formulation is co-amoxiclav, but it is commonly referred to in practice by proprietary names such as Amoksiklav, Augmentin, Clamoxyl, Augclac, Augmexx, and Yucla depending on country.