The combination of amoxicillin and colistin has additive action. Amoxicillin is a semisynthetic broad-spectrum penicillin with a bactericidal action against both Gram-positive and Gram-negative bacteria. The spectrum of amoxicillin includes Campylobacter spp., Clostridium spp., Corynebacterium spp., E. coli, Erysipelothrix spp., Haemophilus spp., Pasteurella spp., Salmonella spp., penicillinase-negative Staphylococcus spp. and Streptococcus spp. The bactericidal action is due to inhibition of cell wall synthesis. Amoxicillin is mainly excreted in urine. A major part can also be excreted in bile. Colistin is an antibiotic from the group of polymyxins with a bactericidal action against Gram-negative bacteria, like E. coli, Haemophilus spp. and Salmonella spp. Since colistin is absorbed for a very small part after oral administration, only gastrointestinal indications are relevant.