The combination of colistin and trimethoprim has additive action. 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. Trimethoprim is an antibacterial agent with a bacteriostatic action against many Gram-negative and Gram-positive bacteria like E. coli, Haemophilus spp., Pasteurella spp., Salmonella spp., Staphylococcus spp. and Streptococcus spp. After oral administration trimethoprim is absorbed almost totally and penetrates well into tissues and cells.