Amoxicillin antibiotic drug, molecular model. This is a moderate-spectrum beta-lactam antibiotic. These drugs work by inhibiting the synthesis of bacterial cell walls, mainly in Gram-positive bacteria. Vulnerable bacteria include Streptococcus and Staphylococcus, and some Gram- negative bacteria such as Neisseria and Escherichia coli. Some strains of previously- susceptible bacteria have developed resistance to amoxicillin, as they produce the enzyme beta- lactamase, which destroys the drug. Hence, it is usually taken with clavulanic acid, which blocks this enzyme, a preparation called co-amoxiclav. Atoms are colour-coded: carbon (green), nitrogen (blue), oxygen (pink), sulphur (orange) and hydrogen (white).