Fusidic acid is an effective treatment against Toxoplasma gondii and Listeria monocytogenes in vitro, but not in mice.

Fusidic acid is a bacteriostatic antibiotic that inhibits the growth of bacteria by preventing the release of translation elongation factor G (EF-G) from the ribosome. The apicomplexan parasite Toxoplasma gondii has an orthologue of bacterial EF-G that can complement bacteria and is necessary for parasite virulence. Fusidic acid has been shown to be effective in tissue culture against the related pathogen Plasmodium falciparum, and current drug treatments against T. gondii are limited. We therefore investigated the therapeutic value of fusidic acid for T. gondii and found that the drug was effective in tissue culture, but not in a mouse model of infection. To determine whether this trend would occur in another intracellular pathogen that elicits a T helper 1-type immune response, we tested the efficacy of fusidic acid for the bacterium Listeria monocytogenes. Similar to its effects on T. gondii, fusidic acid inhibits the growth of L. monocytogenes in vitro, but not in mice. These findings highlight the necessity of in vivo follow-up studies to validate in vitro drug investigations.