Antimicrobial susceptibility of Legionella gormanii: species-specific ECOFFs, distinct MICs for rifampin versus Legionella pneumophila.

Background: Legionella gormanii (L. gormanii) is an emerging pathogen causing legionellosis, yet it is much less studied than the predominant species, L. pneumophila. Clinical treatment guidelines for legionellosis are primarily based on data from L. pneumophila and recommend macrolides and fluoroquinolones. However, the intrinsic antimicrobial susceptibility of L. gormanii is not well-defined, creating uncertainty about whether these guidelines are directly applicable. Establishing a baseline understanding of its susceptibility patterns is a prerequisite for effective epidemiological surveillance and for identifying non-wild-type resistance. This study aims to address this fundamental knowledge gap by characterizing the in vitro susceptibility profiles of a large collection of environmental L. gormanii isolates.

Methods: This study systematically evaluated the in vitro activity of ten representative antimicrobials against 207 environmental L. gormanii isolates collected in China between 2019 and 2021. Minimum inhibitory concentrations (MICs) were determined by the broth microdilution (BMD) method, and species-specific epidemiological cutoff values (ECOFFs) were established using the ECOFFinder tool.

Results: Most tested agents demonstrated good in vitro activity. Rifampin was the most potent agent, while clarithromycin was the most active macrolide. Conversely, tetracyclines showed limited activity. Comparative analysis revealed that L. gormanii exhibited markedly higher MICs for rifampin (approximately 9.58-fold) than typically reported for L. pneumophila. Species-specific ECOFFs were determined for nine antimicrobials: rifampin (0.016 mg/L); ciprofloxacin, levofloxacin, and clarithromycin (0.063 mg/L); moxifloxacin (0.125 mg/L); erythromycin (0.25 mg/L); azithromycin (0.5 mg/L); trimethoprim-sulfamethoxazole (4 mg/L); and tigecycline (16 mg/L).

Conclusions: This study establishes the first large-scale susceptibility dataset and species-specific ECOFFs for L. gormanii. The findings highlight key inter-species differences in susceptibility, particularly for rifampin, underscoring that treatment paradigms cannot be safely extrapolated from L. pneumophila. These ECOFFs provide a critical tool for surveillance of non-wild-type resistance. However, these data, derived from environmental isolates, are intended for epidemiological and hypothesis-generating purposes and must be supplemented with clinical and pharmacokinetic/pharmacodynamic (PK/PD) studies before informing changes to clinical practice.