Antimicrobial susceptibility testing reveals reduced susceptibility to azithromycin and other antibiotics in Legionella pneumophila serogroup 1 isolates from Portugal

BackgroundAlthough not fully investigated, studies show that Legionella pneumophila can develop antibiotic resistance. As there is limited data available for Portugal, we determined the antibiotic susceptibility profile of Portuguese L. pneumophila serogroup 1 (LpnSg1) isolates against antibiotics used in the clinical practice in Portugal. MethodsMinimum inhibitory concentrations (MICs) were determined for LpnSg1 clinical (n=100) and related environmental (n=7) isolates, collected between 2006-2022 in the context of the National Legionnaires Disease Surveillance Programme, against azithromycin, clarithromycin, erythromycin, levofloxacin, ciprofloxacin, moxifloxacin, rifampicin, doxycycline, tigecycline, and amoxicillin/clavulanic acid, using 3 different assays. Isolates were also PCR-screened for the presence of the lpeAB gene. ResultsTwelve isolates had azithromycin MICs above the EUCAST tentative highest WT MIC, 9 of which were lpeAB negative; for erythromycin and clarithromycin, all isolates tested within the susceptible range. The number of isolates with MICs above the tentative highest WT MIC for the remaining antibiotics was: ciprofloxacin: 7; levofloxacin: 17; moxifloxacin: 8; rifampicin: 11; doxycycline: 82; tigecycline: 4. EUCAST breakpoints are not available for amoxicillin/clavulanic acid. We estimated the ECOFFs and one isolate had a MIC 8-fold higher than the E-test ECOFF. Additionally, a clinical isolate generated three colonies growing on the E-test inhibition zone that resulted in MICs 4-fold higher than for the parental isolate. ConclusionsWe report, for the first time, elevated MICs against first-line and other antibiotics (including azithromycin, fluoroquinolones and amoxicillin/clavulanic acid commonly used to treat pneumonia patients in Portugal) in Portuguese L. pneumophila strains. Results point towards decreased susceptibility in circulating strains, justifying further investigation.