Doxycycline belongs to the group of tetracyclines and acts bacteriostatic against many Gram-positive and Gram-negative bacteria like Bordetella, Campylobacter, E. coli, Haemophilus, Pasteurella, Salmonella, Staphylococcus and Streptococcus spp. Doxycycline is also active against Chlamydia, Mycoplasma and Rickettsia spp. The action of doxycycline is based on inhibition of bacterial protein synthesis. Doxycycline has good affinity towards the lungs and is therefore especially useful for treatment of bacterial Respiratory infections. Colistin is an antibiotic from the group of polymyxins with bactericidal action against mainly Gram-negative bacteria like E. coli, Haemophilus and Salmonella spp. Polymyxins interact strongly with phospholipids and penetrate into and disrupt the structure of cell membranes. Administered orally, colistin is poorly resorbed and, therefore, performs its action predominantly gastrointestinally.