Doxycycline is a broad spectrum antibiotic with bacteriostatic action against many gram positive and gram negative organisms such as: Staphylococcus spp., Streptococcus spp., Haemophilus influenza, E.coli, clostridium tetani,listeria,salmonella spp., as well as mycoplasma,Chlamydia and rickettsia.
Doxycycline belong to tetracycline group and has advantage over the older tetracycline of longer serum half-life due to the slow excretion and the enhanced oral absoription.