Propionibacterium spp. are Gram-positive, slow-growing anaerobic, non-spore-forming rods. They are part of the normal flora of the skin, nasopharynx, oral cavity, and genitourinary tract. Generally thought to be nonpathogenic in humans, they can cause infective endocarditis, especially in carriers of mechanical heart valves and pacemakers or implantable cardioverter defibrillators (ICDs) [
8]. This may be due to their ability to adhere to foreign body surfaces and produce a biofilm. In a series of 58 patients with infective endocarditis due to
Propionibacterium spp. published in 2009, prosthetic valve endocarditis occurred in 67 % of the patients and was the most common presentation of
Propionibacterium spp. infection. In addition to antimicrobial treatment, 81 % of the patient underwent cardiac surgery (due to the propensity of the organism to form an abscess [
9]), and the overall mortality rate was 16 %[
8]. The patient described above was cured with antibiotic therapy alone. Most of the infections of mechanical heart valves occur at the time of surgery. With high inocula, a short incubation period is expected. However,
P. acnes may reside intracellularly in macrophages and remain there dormant for years [
10]. In our patient, the infection may also have occurred during the frequent skin punctures necessary for the determination of the level of anticoagulation [
11].
Propionibacterium endocarditis may be difficult to diagnose. The median incubation period required for blood cultures to become positive is 7 days (range 5–14 days) [
2]. Therefore, a prolonged incubation period of two weeks is proposed in cases of suspected infection with
Propionibacterium spp. Additional diagnostic methods are often necessary. PCR to detect 16S rDNA in blood is used in cases of blood culture-negative infective endocarditis. However, its sensitivity is lower than that of blood culture methods, and blood specimens need to be pretreated to remove background bacterial DNA contamination [
12]. Gas-liquid chromatographic examination of subcultured bacterial colonies may assist in the detection of anaerobic bacteria [
13]. In blood culture-negative infective endocarditis, which occurs in 2.5–31 % of all cases of infective endocarditis, serologic techniques may be useful [
14], and are established to diagnose infections including
Brucella spp.,
Coxiella burnetii [
5],
Bartonella spp.,
Mycoplasma spp., and
Legionella spp.[
15]. In the patient described above, an antibody response to
P. acnes was quantified (Fig.
1), showing a titer of 1:8 at the time of diagnosis and a gradual decline thereafter. Therefore, this antibody could potentially facilitate the early diagnosis of infective endocarditis due to
P. acnes. When uncertainty exists regarding whether the growth of
P. acnes in blood cultures is a contaminant, as occurs in > 95 % of cases [
1], or whether it has a true pathogenetic role, a detectable antibody titer would support the latter. However, increased antibody titers against
P. acnes have been reported in diseases such as acne vulgaris [
16] or benign prostatic hyperplasia [
4], lowering their specificity. In patients with acne a time course of the serology can detect a rising antibody titer suggesting
P. acnes infection.
Treatment of
P. acnes infective endocarditis primarily consists of a β-lactam antibiotic, often combined with an aminoglycoside [
8], even though P. acnes is frequently resistant to the latter [
17]. Oral rifampicin must be added because of its ability to penetrate the bacterial biofilm [
8] and may be combined with a chinolone antibiotic [
18]. In vitro data suggest that the combination of daptomycin with rifampicin is highly active against
P. acnes biofilms [
19]. Long-term antibiotic therapy is required for the successful treatment of infectious endocarditis due to
P. acnes. The treatment effect is mirrored by the decreasing antibody titer.