Tiasan-100 is a semisynthetic derivative of the naturally occurring diterpene antibiotic pleuromutilin with bacteriostatic action against Gram-positive bacteria (e.g. staphylococci, streptococci, Arcanobacterium pyogenes), Mycoplasma spp. spirochetes (Brachyspira hyodysenteriae, B. pilosicoli) and some Gram-negative bacilli such as Pasteurella spp. Bacteroides spp. Actinobacillus (Haemophilus) spp. Fusobacterium necrophorum, Klebsiella pneumoniae and Lawsonia intracellularis. Tiamulin distributes widely in tissues, including the colon and the lungs, and acts by binding to the 50S ribosomal subunit, thereby inhibiting bacterial protein synthesis.