Effectiveness of ciprofloxacin or linezolid in combination with rifampicin against Enterococcus faecalis in biofilms

Objectives: To assess the in vitro antibiotic susceptibility of Enterococcus faecalis in biofilms. Methods: E. faecalis isolates were from infections of prosthetic hip and knee joints. Minimal biofilm eradication concentrations (MBECs) for ampicillin, vancomycin, linezolid, ciprofloxacin and rifampicin, alone and in combinations, were determined. E. faecalis biofilms in microtitre plate wells or on beads of bone cement were treated with the same antibiotics for various times followed by plating to quantify surviving bacteria. Results: The E. faecalis isolates displayed MBECs for the various antibiotics of between 64 and 512 mg/L. When combined with 8 mg/L rifampicin, MBECs for ciprofloxacin and linezolid dropped to 16-32 mg/L for the four isolates tested. In biofilms formed on a plastic surface and on bone cement beads significant bacterial killing (. 23 log decrease in cfus) was seen after an 8 h exposure to antibiotics. For all isolates, the combination of ciprofloxacin and rifampicin, followed by linezolid and rifampicin, was most effective in reducing the number of bacteria. E. faecalis developed resistance to rifampicin in our system when biofilms were subjected to this antibiotic, alone or in combination with ampicillin. When bacteria were subjected to combinations with ciprofloxacin or linezolid development of resistance was reduced. Conclusions: Our results show that combinations of ciprofloxacin or linezolid with rifampicin have a good effect on E. faecalis biofilms in vitro. These combinations should be considered for testing in humans with early joint prosthesis infections with E. faecalis.