Occurrence of macrolides resistance in Legionella pneumophila ST188: results of the Belgian epidemiology and resistome investigation of clinical isolates.

Introduction: The incidence of Legionnaires' disease (LD) steadily increases worldwide. Although Legionella pneumophila is known as pathogenic, systematic investigations into antibiotic resistance are scarce, and reports of resistance in isolates are recently emerging.

Methods: Clinical cases and metadata reported to the Belgian National Reference Centre between 2011 and 2022 were retrospectively analysed. 283 clinical isolates were typed by cgMLST. Acquired genes or mutations triggering resistance were extracted from all of them.

Results: The number of LD cases has increased in Belgium. Urinary antigen testing remains the main used test, but PCR and serology allow the diagnostic in 14.8% and 2.4% of cases. cgMLST showed a good discrimination between STs and minimal variation for ST47 isolates, while ST1's were more diverse. Genotypic screening identified a 23S rRNA mutation linked to a high-level macrolide resistance in one isolate of ST188, which is genetically closed to resistant isolates from France.

Conclusion: The raise in incidence is of concern and is likely an under-estimate due to the reliance on urine antigen testing. Routine typing by cgMLST allows good discrimination and the first clinical isolate reported as resistant for macrolides was cultured, underscoring the need to define resistance breakpoints and incorporate AST as routine clinical investigation practice.