We report the case of an 85-year-old woman with a history of hypertension, diabetes mellitus and permanent pacemaker implantation, who underwent bioprosthetic aortic valve replacement (Sorin Crown PRT 19mm) due to aortic stenosis. Eight months later the patient presented at the emergency room with fatigue, malaise and weight loss over a period of 8 weeks. She was febrile (38.3°C) and presented clinical and radiologic features of heart failure. Laboratory test revealed anemia (Hb 8.2g/dL), leukocytosis (14,800/mcL), and C reactive protein was 86mg/L. Electrocardiogram showed a complete atrioventricular block with normal pacemaker stimulation. Two sets of blood cultures were obtained. The patient was admitted with a diagnosis of viral respiratory infection and congestive heart failure. Antibiotic therapy was not initiated. Twenty-four hours after admission she presented a ventricular tachycardia without pulse requiring electric cardioversion. A transthoracic echocardiogram showed a severe prosthetic aortic valve stenosis with calcified unstructured leaflets and thickened calcified mitral leaflets, without images of vegetations. For the next three days the patient remained afebrile but congestive heart failure worsened. On day 4 transesophageal echocardiogram (TEE) revealed a vegetation (4mm) in the aortic prosthesis and a possible perivalvular abscess; after that an Infectious Disease specialist was consulted. Three further sets of blood cultures were obtained, empirical therapy was started with daptomycin, ampicillin and ceftriaxone, and surgery was recommended. On day 3, coryneform Gram-positive bacilli grew in anaerobic bottles of blood cultures collected at admission (after 65h of culture). This result was initially regarded as a contamination and was not reported to clinicians till the next day when the microbiologist was informed about the infective endocarditis (IE) suspicion. These bacteria were initially identified as Propionibacterium spp., and subsequently as P. avidum by MALDI-TOF. On day 8 all three sets of blood cultures obtained before antibiotic therapy was initiated were growing coryneform Gram-positive bacilli in anaerobic bottles. So Propionibacterium spp. were considered the responsible for the IE and an antibiogram-guided de-escalation was made to penicillin G (24 million IU/24h). The patient underwent surgery on day 11. Unfortunately the postoperative course was complicated by refractory shock and multi-organic failure, and the patient died 48h after surgery.
Propionibacterium spp. are ubiquitous Gram-positive coccobacilli which are constituents of human skin microflora. Major species include P. acnes, P. granulosum and P. avidum. Traditionally considered to be species of low virulence, P. acnes and to a lesser extent P. granulosum have been implicated in serious infections, including IE.1•5P. avidum has been reported as an overlooked cause of breast abscesses after surgery, of septic arthritis after intraarticular treatment, and of abscesses in other locations after surgery or invasive procedures.1,6 However, to the best of our knowledge this is the third case of IE due to P. avidum reported in the literature, after the ones reported by Vetromile7 and Braun.8
The incidence of Propionibacterium spp. IE is reported to be 0.3•1.4 cases per year.2 As in our case, Propionibacterium spp. is usually associated with a long history of minimal clinical signs of infection.3 It involves prosthetic valves in 67% of cases and abscess formation occurs in 36% of cases, specially involving prosthetic cardiac devices.3
In Propionibacterium spp. IE the reported median growing time in blood cultures is 7 days.3 Therefore, blood samples should be cultured for more than 7 days before a diagnosis of bacteremia can be rejected,2,3,5 especially when IE is suspected, in order to permit the growth of fastidious microorganisms other than the HACEK group bacteria like Propionibacterium spp. We stress that only a minority of Propionibacterium spp. bacteremias are clinically significant.9 The clinical context and the number of positive bottles are essential to an assessment of the real significance of Propionibacterium spp. bacteremias, but microbiologists should be alert to the possibility of IE.
Propionibacterium spp. are highly susceptible to a wide range of antibiotics, including beta-lactams, quinolones, clindamycin and rifampin, but not metronidazole. In absence of consensus on the management of Propionibacterium spp. IE, most physicians choose a 6-week course of a beta-lactam antibiotic, alone or in combination with aminoglycosides.2,3 Surgery is needed in a high proportion of patients, mainly due to perivalvular extension and abscess formation.3,10 Although Propionibacterium spp. have low virulence, the overall mortality rate in Propionibacterium spp. IE is relatively high (16•27%) due to the frequent valvular and perivalvular destruction.2,3
To conclude, this case emphasizes a number of important concepts. First, both clinicians and microbiologists should be aware of the possibility of Propionibacterium spp. (even other than P. acnes) as a possible etiology of IE, especially in view of its significant mortality. Second, blood samples should be cultured for prolonged periods of time in order to permit the identification of fastidious microorganisms like Propionibacterium spp. Third, permanent feedback between microbiologists and clinicians is essential to ensure that significant information is not missed.
FundingNo funding was required for this work.
Conflict of interestThe authors declare no conflict of interest.
We are grateful to Nuria Fernández Hidalgo and Joaquim Martínez-Montauti for their critical review, to Francesc Marco for the MALDITOF identification, and to Michael Maudsley for English language support and editorial assistance.
