In vitro activity effects of twelve antibiotics alone and in association against twenty-seven Enterococcus faecalis strains isolated from Italian patients with infective endocarditis: high in vitro synergistic effect of the association ceftriaxone-fosfomycin.

BACKGROUND: In 2004-2008, the epidemiological and clinical Infective Endocarditis Study Group (SEI) evaluated 852 cases of infective endocarditis. Staphylococcus aureus was the main involved pathogen (24.5%) and Enterococcus faecalis etiology was described in 11% of the cases. The aim of this study was to evaluate the in vitro activity of 12 antibiotics alone and in association against 27 strains of E. faecalis isolated from blood cultures of patients with infective endocarditis.

RESULTS: The results showed high in vitro activity of tigecycline, daptomycin and linezolid. A high synergistic effect was obtained with the association ceftriaxone-fosfomycin [fractional inhibitory concentration (FIC)(50) = 0.34, FIC(90) = 0.78]. Furthermore, ceftriaxone plus ampicillin presented additive results (FIC(50) = 0.66, FIC(90) = 1.00), and ceftriaxone plus fosfomycin and ceftriaxone plus ampicillin were significantly more active in vitro than each drug alone. The efficacy of ceftriaxone plus fosfomycin was confirmed by the association testing using the broth dilution technique.

CONCLUSION: Fosfomycin seems particularly significant and its association with ceftriaxone could be considered as a useful therapeutic option in medical treatment of E. faecalis infective endocarditis.