KKL-35 exhibits potent antibiotic activity againstLegionellaspecies independently of trans-translation inhibition

Abstract

AbstractTrans-translation is a ribosome rescue system that is ubiquitous in bacteria. Small molecules defining a new family of oxadiazole compounds that inhibit trans-translation have been found to have broad-spectrum antibiotic activity. We sought to determine the activity of KKL-35, a potent member of the oxadiazole family, against the human pathogenLegionella pneumophilaand other related species that can also cause Legionnaires disease (LD). Consistent with the essential nature of trans-translation inL. pneumophila, KKL-35 inhibits growth of all tested strains at sub-micromolar concentrations. KKL-35 is also active against other LD-causingLegionellaspecies. KKL-35 remains equally active againstL. pneumophilamutants that have evolved resistance to macrolides. KKL-35 inhibits multiplication ofL. pneumophilain human macrophages at several stages of infection. No resistant mutants could be obtained, even during extended and chronic exposure. Surprisingly, KKL-35 is not synergistic with other ribosome-targeting antibiotics and does not induce the filamentation phenotype observed in cells defective for trans-translation. Importantly, KKL-35 remains active againstL. pneumophilamutants expressing an alternate ribosome-rescue system and lacking tmRNA, the essential component of trans-translation. These results indicate that the antibiotic activity of KKL-35 is not related to the specific inhibition of trans-translation and its mode of action remains to be identified. In conclusion, KKL-35 is an effective antibacterial agent against the intracellular pathogenL. pneumophilaand with no detectable resistance. However, further studies are needed to better understand its mechanism of action and to assess further the potential of oxadiazoles in treatment.