In vitro activity of linezolid against clinical isolates of Fusobacterium spp.

OBJECTIVES Although most susceptibility studies for linezolid have investigated aerobic bacteria, only a few have investigated anaerobe isolates. The aim of the present study was to determine the antibacterial activity of linezolid against a larger sample of clinical isolates of Fusobacterium spp. and to report on the detailed susceptibility, stratified by species. METHODS The in vitro susceptibility of 80 clinical isolates of Fusobacterium (Fusobacterium necrophorum, n = 34; Fusobacterium nucleatum, n = 20; Fusobacterium varium, n = 18; Fusobacterium mortiferum; n = 8) was tested and compared with the activity of the older compounds amoxicillin and amoxicillin/clavulanic acid. RESULTS The MIC of linezolid ranged from 0.016 to 1.0 mg/L, with the MIC(90) being 0.5 mg/L. The highest MIC obtained for linezolid (1.0 mg/L) was measured for an F. varium isolate. The MIC(90) for both, amoxicillin (range: 0.016-0.75 mg/L) and amoxicillin/clavulanic acid (range: 0.047-0.75 mg/L), was 0.5 mg/L. Overall, no resistant strains were found in the study. CONCLUSIONS Compared with amoxicillin and amoxicillin/clavulanic acid, linezolid was less active against F. necrophorum (MIC(90) 0.25 mg/L) and F. nucleatum (MIC(90) 0.25 mg/L), equally active against F. varium (MIC(90) 0.75 mg/L) and slightly more active against F. mortiferum (MIC(90) 0.19 mg/L).