Antimicrobial Susceptibility Determination of Less Frequently Isolated Legionella Species by Broth and Agar Dilution
Abstract
Background/Objectives: Infections caused by Legionella species are primarily associated with Legionella pneumophila, but non-pneumophila species are increasingly implicated in human disease. Despite this, antimicrobial susceptibility testing (AST) data for non-pneumophila species remain scarce, and standardised testing protocols or resistance thresholds have not been established. This study aimed to address this gap by evaluating and comparing AST performance for non-pneumophila Legionella species relative to L. pneumophila using three methodologies. Methods: AST was conducted on 89 Legionella isolates using LASARUS agar dilution, buffered yeast extract broth microdilution (BYE-BMD), and BCYE-α agar dilution, against ampicillin, azithromycin, chloramphenicol, doxycycline, levofloxacin, and rifampicin. Growth performance and minimum inhibitory concentrations (MICs) were assessed after a 96 h incubation. Results: MIC profiles were obtained using LASARUS and BYE-BMD for 53.9% and 93.3% of isolates, respectively. While L. pneumophila reached sufficient turbidity in BYE-BMD after a 48 h incubation, non-pneumophila species required an extended incubation (72-96 h). Non-pneumophila species displayed broader MIC ranges against azithromycin (0.016-1 mg/L) and levofloxacin (0.016-0.25 mg/L), but a narrower rifampicin range (≤0.0005-0.032 mg/L) relative to L. pneumophila. L. longbeachae exhibited a higher MIC50 for rifampicin despite overlapping susceptibility ranges across all species…
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