In vitro susceptibility to antibiotics of staphylococci in biofilms isolated from orthopaedic infections.

Prosthetic joint infections (PJIs) are related to the formation of biofilms, mainly by Staphylococcus aureus and Staphylococcus epidermidis. Therapy is usually selected according conventional susceptibility testing, but these data may be insufficient to detect the true antibiotic susceptibility in the biofilm. In total, 32 clinical strains (17 S. aureus and 15 S. epidermidis) isolated from patients with PJIs as well as 2 collection strains (S. aureus 15981 and S. epidermidis ATCC 35984) were tested against nine antibiotics commonly used in the treatment of PJIs (rifampicin, vancomycin, tigecycline, clindamycin, trimethoprim/sulfamethoxazole, ciprofloxacin, cloxacillin, daptomycin and fosfomycin) using the Calgary Biofilm Device. None of the antibiotics proved to be totally effective against biofilms in both species, with minimum biofilm eradication concentrations (MBECs) highly above the minimum inhibitory concentrations for most of the antibiotics (>1024 mg/L). Rifampicin and tigecycline showed MBECs slightly lower, mainly against S. epidermidis biofilms, and only two strains of this staphylococcal species were susceptible to almost all of the antibiotics tested. These results show that the search for new compounds with antimicrobial and antibiofilm properties is mandatory as well as the development of other strategies that could lead to the prevention and treatment of PJIs. In addition, more studies are necessary to obtain a better understanding of the mechanisms involved in antimicrobial resistance of biofilms.