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 MIC90 being 0.5 mg/L. The highest MIC obtained for linezolid (1.0 mg/L) was measured for an F. varium isolate. The MIC90 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 (MIC90 0.25 mg/L) and F. nucleatum (MIC90 0.25 mg/L), equally active against F. varium (MIC90 0.75 mg/L) and slightly more active against F. mortiferum (MIC90 0.19 mg/L).