Role of rifampin against Propionibacterium acnes biofilm in vitro and in an experimental foreign-body infection model

Abstract

Propionibacterium acnes is an important cause of orthopedic-implant-associated infections, for which the optimal treatment has not yet been determined. We investigated the activity of rifampin, alone and in combination, against planktonic and biofilm P. acnes in vitro and in a foreign-body infection model. The MIC and the minimal bactericidal concentration (MBC) were 0.007 and 4 μg/ml for rifampin, 1 and 4 μg/ml for daptomycin, 1 and 8 μg/ml for vancomycin, 1 and 2 μg/ml for levofloxacin, 0.03 and 16 μg/ml for penicillin G, 0.125 and 512 μg/ml for clindamycin, and 0.25 and 32 μg/ml for ceftriaxone. The P. acnes minimal biofilm eradication concentration (MBEC) was 16 μg/ml for rifampin; 32 μg/ml for penicillin G; 64 μg/ml for daptomycin and ceftriaxone; and ≥128 μg/ml for levofloxacin, vancomycin, and clindamycin. In the animal model, implants were infected by injection of 10⁹ CFU P. acnes in cages. Antimicrobial activity on P. acnes was investigated in the cage fluid (planktonic form) and on explanted cages (biofilm form). The cure rates were 4% for daptomycin, 17% for vancomycin, 0% for levofloxacin, and 36% for rifampin. Rifampin cured 63% of the infected cages in combination with daptomycin, 46% with vancomycin, and 25% with levofloxacin. While all tested antimicrobials showed good activity against planktonic P. acnes, for eradication of biofilms, rifampin was needed. In combination with rifampin, daptomycin showed higher cure rates than with vancomycin in this foreign-body infection model.

Fig 1

Heat production of P. acnes in biofilm. Heat was produced by biofilm bacteria on sintered glass beads after 3 h, 24 h, and 72 h of biofilm formation (in duplicate). The dashed line marks the detection limit of 10 μW. A sterile bead served as a negative control.

Fig 2

MBECs determined by microcalorimetry. Shown is heat produced by recovering biofilm P. acnes after a 24-h exposure to serial dilutions of rifampin (A), daptomycin (B), levofloxacin (C), vancomycin (D), clindamycin (E), penicillin G (F), and ceftriaxone (G). The MBECs are circled. The detection limit was 10 μW.

Fig 3

Infection profile of P. acnes in a foreign-body guinea pig infection model. Shown are bacterial loads in aspirated cage fluid during 50 days of infection and the percentages of culture-positive explanted cages after 16 days and 50 days of infection using a low infection inoculum of 5 × 10⁷ CFU/cage (A) and a high infection inoculum of 1 × 10⁹ CFU/cage (B). The values are means ± standard deviation.

Fig 4

Treatment efficacy against planktonic P. acnes. Shown are the bacterial loads in cage fluid aspirated during treatment (white bars) and 5 days after treatment (black bars). The values are means and standard deviations (SD). DAP, daptomycin; VAN, vancomycin; LEVO, levofloxacin; RIF, rifampin. The treatment groups are compared to the control during treatment and after treatment. *, P < 0.05; **, P < 0.01; ***, P < 0.005; ns, not significant.

Fig 5

Treatment activity against biofilm P. acnes. Shown are the cure rates of adherent bacteria from explanted cages. The percentages above the columns indicate the cure rates. *, P < 0.05; **, P < 0.01; ***, P < 0.005.