Activity: A broad spectrum antimicrobial drug. Florfenicol is a broad-spectrum, primarily bacteriostatic, antibiotic with a range of activity similar to that of chloramphenicol, including many gram-negative and gram-positive organisms; however,florfenicol does not carry the risk of inducing human aplastic anemia that is associated with chloramphenicol. Florfenicol has been demonstrated to be active in vitro and in vivo against Mannheimia (Pasteurella) haemolytica, Pasteurella multocida, and Haemophilus somnus. In vitro studies have demonstrated florfenicol activity against Enterobacter cloacae, Escherichia coli, Klebsiella pneumoniae, Salmonella typhi, and Shigella dysenteriae,but with at least a 2- to 10-fold higher minimum inhibitory
concentration than that for the Mannheimia, Pasteurella and Haemophilus species listed above. It also has activity against some chloramphenicol-resistant strains of bacteria,possibly because it is less affected by the major enzyme produced in plasmid-mediated bacterial resistance against chloramphenicol and thiamphenicol. Although the activity of florfenicol against obligate anaerobes is not addressed in the literature, it is likely to be quite effective.