AMSTERDAM (FRONTLINE MEDICAL NEWS) – Accounting for less than 1% of cases, Propionibacterium acnes has been considered an uncommon cause of infective endocarditis.

But data presented at the annual congress of the European Society of Clinical Microbiology and Infectious Diseases suggest that the common anaerobe may be responsible for many more cases than is now believed. The bacteria are difficult to culture and grow very slowly, Dr. Jona Banzon said at the meeting. Incubating it for the standard 5 days may simply not be long enough.

“Due to this slow-growing nature, the standard incubation period may not be enough to detect it,” said Dr. Banzon, an infectious disease fellow at the Cleveland Clinic. “And since P. acnes is part of our commensal flora, it’s frequently a contaminate in culture, and we might be inappropriately dismissing it as such. In fact, we now wonder if this lack of extended incubation may be accounting for a significant proportion of negative blood cultures, and whether more prosthetic valve endocarditis than we think is actually being caused by P. acnes.”

Dr. Banzon presented a series of 23 cases included in the Cleveland Clinic Infective Endocarditis Registry from the period of 2007-2015. All had P. acnes confirmed as the causative organism. The group comprises 3.3% of the institution’s entire infective endocarditis registry, “making infective carditis with P. acnes already much more common than it is said to be in the literature,” she noted.

All of the cases were confirmed by any of the following standards:

• Two or more blood cultures positive for P. acnes.

• Two or more valve cultures positive for P. acnes.

• Two or more valve sequencing rests positive for P. acnes.

• At least two of the following: a positive blood culture, a positive valve culture or valve sequencing, or histopathologic demonstration of microorganism consistent with P. acnes.

Of the cohort, 22 had prosthetic valve endocarditis. One patient had endocarditis on a native valve. This is an important point, Dr. Banzon said.

The organism is being increasingly recognized for causing infections of prosthetic material, including shoulder joint infections and shunts, but it rarely seems to affect native tissue. The patient who had native valve endocarditis had experienced an episode of Staphylococcus aureus endocarditis about 18 months earlier. He was not treated surgically, and ended up with a damaged valve. After the initial illness, he was readmitted several times with symptoms of endocarditis, but all of his blood cultures were negative. This was attributed to the receipt of antibiotics. “When he was finally operated on, there were three valve sequencing specimens and all were positive for P. acnes,” Dr. Banzon said.

The patients in the cohort were a mean of 74 years old; about 75% were men. All of the cases were left-sided endocarditis, with the majority (74%) involving the aortic valve. Other sites included the mitral valve (18%), aortic plus mitral (4%), and aortic plus tricuspid (4%).

The most common predisposing factor was having a prosthetic valve (96%). Other factors included having a cardiac implantable device (17%), and a prior episode of infective endocarditis (13%). None of the patients had indwelling vascular catheters or used injectable drugs.

The cases presented with severe disease, Dr. Banzon said. Almost half (48%) had a perivalvular abscess at presentation. A third (35%) had valve dehiscence, and 35% had severe valvular regurgitation. There was a vegetation of more than 1 cm in 9% of cases.

Emboli were not uncommon; 17% had emboli in the central nervous system, and 17% had peripheral emboli. Two of these patients had kidney and spleen infarcts and two had acute arterial thromboembolism that required thrombectomy.

All of the patients underwent blood cultures; overall, 30% of the cultures were positive for P. acnes. But there were “striking differences” when the tests were broken down by incubation time, Dr. Banzon said.

Most of the cultures (16) were incubated for the standard of 5 days or less. Among these, 12.5% were positive. However, cultures on seven patients were incubated for more than 5 days and in this group, 71.4% were positive for P. acnes. The median time to positivity overall was 7 days, with a range of 3-9 days.

Other diagnostic methods were important in closing this gap, she said. Valve culturing was positive in 57% of the cases, while valve sequencing was positive in 95%. The median time to positive for valve culture was somewhat shorter than for blood culture (5.5 days).

In nine cases, no organism would have been identified without valve sequencing, Dr. Banzon said.

Because they presented with severe disease, almost all of the patients (22) underwent surgery as their intimal treatment. At the time of surgery, everyone was taking an antibiotic that covered P. acnes. Single-agent therapy was the definitive treatment for most, with vancomycin being most commonly employed (59%), followed by ceftriaxone (25%). A few patients had a combination of both drugs or a combination of vancomycin and rifampin. One patient took penicillin.

The single patient who was medically treated received 6 weeks of intravenous ceftriaxone. After 1 month, he was readmitted with blood cultures positive for P. acnes. He underwent surgery and a valve sequencing confirmed P. acnes as the infective agent.

There were two in-hospital deaths, but the rest of the patients were discharged on antibiotic therapy and recovered with no additional deaths or relapses.

The extended time P. acnes required to show in culture was enough for the Cleveland Clinic to reconsider incubation guidelines for the microorganism, Dr. Banzon said.

“There are enough cases taking 9 or 10 days that we now always hold these cultures for at least 10 days when we’re looking for P. acnes.”

She had no financial disclosures.