KKL-35 Exhibits Potent Antibiotic Activity against Legionella Species Independently of <i>trans</i> -Translation Inhibition

Abstract

<i>trans</i>-Translation is a ribosome-rescue system that is ubiquitous in bacteria. Small molecules defining a new family of oxadiazole compounds that inhibit <i>trans</i>-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 pathogen <i>Legionella pneumophila</i> and other related species that can also cause Legionnaires' disease (LD). Consistent with the essential nature of <i>trans</i>-translation in <i>L. pneumophila</i>, KKL-35 inhibited the growth of all tested strains at submicromolar concentrations. KKL-35 was also active against other LD-causing <i>Legionella</i> species. KKL-35 remained equally active against <i>L. pneumophila</i> mutants that have evolved resistance to macrolides. KKL-35 inhibited the multiplication of <i>L. pneumophila</i> in human macrophages at several stages of infection. No resistant mutants could be obtained, even during extended and chronic exposure. Surprisingly, KKL-35 was not synergistic with other ribosome-targeting antibiotics and did not induce the filamentation phenotype observed in cells defective for <i>trans</i>-translation. Importantly, KKL-35 remained active against <i>L. pneumophila</i> mutants expressing an alternate ribosome-rescue system and lacking transfer-messenger RNA, the essential component of <i>trans</i>-translation. These results indicate that the antibiotic activity of KKL-35 is not related to the specific inhibition of <i>trans</i>-translation and its mode of action remains to be identified. In conclusion, KKL-35 is an effective antibacterial agent against the intracellular pathogen <i>L. pneumophila</i> with no detectable resistance development. However, further studies are needed to better understand its mechanism of action and to assess further the potential of oxadiazoles in treatment.