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 the inhibition of bacterial protein synthesis. Doxycycline has a good affinity towards the lungs and is therefore especially useful for the treatment of bacterial respiratory infections. Gentamicin sulphate belongs to the group of aminoglycosides and acts bactericidal against mainly Gram-negative bacteria like E. coli, Klebsiella, Pasteurella, and Salmonella spp. The bactericidal action is based on the inhibition of bacterial protein synthesis as well. Administered orally, gentamicin is poorly resorbed and performs its action predominantly in the gastrointestinal tract.