Antibiotics for the eradication of Propionibacterium acnes biofilms in surgical infection

Objectives: Propionibacterium acnes is increasingly recognized as a cause of delayed infection after spinal instrumentation or shunting for hydrocephalus. Biofilm development by this organism has recently been demonstrated. We therefore investigated the effect of two different courses of three antibiotics (penicillin, rifampicin and linezolid) on mature P. acnes biofilms in vitro. Outcomes were eradication or regrowth after withdrawal of antibiotics, simulating successful treatment and relapse. Methods: P. acnes biofilms were grown on titanium discs for 6 days until mature, then exposed to the antibiotics for either 7 or 14 days before sonication and culture. Further, discs were similarly exposed, but after each course, they were reincubated for a further 9 days to check for regrowth. Results: Penicillin, linezolid and linezolid plus rifampicin eradicated P. acnes biofilms after 14 days, but only penicillin had this effect after 7 days. 'Relapse' was prevented only by 14 day courses of penicillin or linezolid plus rifampicin, but not by linezolid alone. Conclusions: For P. acnes spinal instrumentation infections, either penicillin or linezolid plus rifampicin might be equally effective. For shunt infections, as penicillin does not give therapeutic cerebrospinal fluid concentrations, rifampicin plus linezolid might be the treatment of choice. Linezolid alone appears not to be as effective as penicillin against P. acnes biofilms.