Doxycycline is a wide spectrum antimicrobial agent, belongs to tetracyclines group that possess antimicrobial activity by binding to the 30S ribosomal subunit of susceptible organisms. It has a good activity against most mycoplasma, spirochetes, Chlamydia and Rickettsia and against gram-positive bacteria such as staphylococcus, streptococci, Actinomyces spp., Bacillus anthracis, Clostridium perfringens and tetani and Listeria monocytogenes. Among gram-negative bacteria, doxycycline has activity against Bordetella spp., Brucella, Haemophilus spp., Pasturella multocida and Shigella.