Legionella pneumophila and macrolides : from susceptibility testing to characterization of molecular mechanisms conferring resistance ; Legionella pneumophila et macrolides : de l’antibiogramme à la caractérisation des mécanismes moléculaires impliqués dans la résistance

Macrolides, alone or in combination, are recommended as first-line therapy of Legionnaires’ disease. Legionella pneumophila is characterized by a constant in vitro susceptibility to macrolides. Resistant variants have never been detected, in contrast with several therapeutic failures observed in patients receiving an adequate treatment. Legionellae isolation from respiratory secretions is needed for antibiotic susceptibility testing, but the reported culture sensitivity reaches only 24% in France. In this work, we improved Legionellae isolation rate by culture and amoebal co-culture. In a second time, we were interested in hypothetic mechanisms involved in therapeutic failures like antibiotic antagonisms and acquired resistance. We found no antagonistic effects for macrolide/fluoroquinolones and macrolide/rifampin combinations in an intracellular Legionella model, which made irrelevant our first hypothesis. In a last step, we propagated Legionella strains with increasing macrolides concentrations. This procedure provided us high-level macrolides-resistant mutants. We found that mutations in the genes encoding L4 and L22 proteins were correlated with low-level resistance. Mutations in domain V of the 23S rRNA were associated with intermediate or high-level resistance. We found a correlation between the kind of nucleotide substitution or the number of mutated ribosomal operons and the resistance level. The speed and efficiency of in vitro selection of L. pneumophila macrolide-resistant mutants suggest a potential acquisition of macrolide resistance in patients under therapy and emphasize the need to explore clinical Legionella macrolide susceptibility in order to track down resistance ; Les macrolides sont recommandés seuls ou en association dans le traitement des légionelloses. Cependant, malgré une antibiothérapie adaptée et l'absence de souches résistantes décrites, des échecs thérapeutiques sont régulièrement observés. L'isolement de souches de Legionella à partir de prélèvements respiratoires est ...