Infective endocarditis (IE) due to anaerobic bacteria and especially
P. acnes is very rare [
1,
2]: Scant data on incidence and treatment of these infections have been published. In fact, the evidence for treatment of IE due to
P. acnes is based on case reports and reviews of the literature: In a Korean analysis of 522 patients with positive blood cultures with
P. acnes, 18 (3.5%) met criteria for bloodstream infection. Only one patient with a ventricular septal defect presented with infective endocarditis [
7]. A recent prospective study of 16 consecutive cases of
P. acnes prosthetic valve endocarditis indicated that the disease might be acquired during heart surgery or could be related to bacteremia of mucocutaneous origin. Diagnosis was often delayed due to oligosymptomatic manifestation, resulting in symptoms associated with progressive prosthetic valve dysfunction leading to heart failure (62%) at the time of diagnosis. The majoritiy of patients had a favourable course (58.3%) when treated with antibiotics and surgical therapy, all patients who initially received antibiotic treatment alone were later in need of surgical intervention due to relapse. Endocarditis related mortality was 18.7% [
3]. Our patient was successfully treated with antibiotics, but without surgery. Several factors might explain, why this approach was successful: First, ceftriaxone MBC
stat (250 mg/l) determined in our patient with stationary phase bacteria were higher than published peak concentrations in serum (150 mg/l), possibly explaining, potentially in part, the failure of this agent (Table
1) if not combined with rifampin. Bacteria on foreign-bodies (FBI) do not grow exponentially, adhere to the surface and may produce biofilm. These conditions are far away from the conditions used in the test tube to determine MICs. Therefore, we performed MBCs with 4×10
5 CFU
P. acnes under anaerobic condition using phosphate-buffered saline as media [
6]. Among agents evaluated in susceptibility testing in our patient, only the expected rifampin peak levels exceeded MBC
stat, an assay that is predictive for successful antimicrobial therapy for FBI [
6,
8]. Rifampin has an excellent property to kill non-growing, adherent bacteria, and is active against biofilm-forming bacteria, as also shown for
P. acnes[
5]. Second, bacterial biofilm formation is central in the pathogenesis of infections related to foreign material, and
P. acnes has been shown to form biofilm both in vitro and in vivo [
9]. Biofilm formation of 93
P. acnes isolates from either invasive infections or skin of healthy people were analysed in a recent Swedish study. Isolates from deep infections produced biofilm, whereas the skin isolates were poor biofilm-producers, which highlights the importance of biofilm formation in
P. acnes virulence [
10]. Third, long-term treatment is commonly required for implant-associated infection, even when signs, symptoms and laboratory parameters returned to normal range after 2–3 weeks after initiating treatment [
11]. Recently published data of an in vitro- and animal infection model [
5] shows that daptomycin in combination with rifampin followed by levofloxacin and rifampin might be a reasonable antibiotic combination to treat FBI due to
P. acnes.