By Richard R. Watkins, MD, MS, FACP, FIDSA, FISAC

Professor of Internal Medicine, Northeast Ohio Medical University, Rootstown, OH

SYNOPSIS: A retrospective study from France used deoxyribonucleic acid (DNA) detection methods to assess the microbial etiologies of 16 cases of culture-negative infective endocarditis. They identified three cases of Malassezia restricta, a yeast considered a member of the human skin microbiota. Notably, serologic testing cross-reacted between M. restricta and Candida albicans.

SOURCE: Houhamdi-Hammou L, Benito Y, Boibieux A, et al. Malassezia restricta: An underdiagnosed causative agent of blood culture-negative infective endocarditis. Clin Infect Dis 2021;73:1223-1230.

Despite advances in laboratory microbial detection methods during the past decade, culture-negative infective endocarditis (CNIE) still is a frequent clinical conundrum. Houhamdi-Hammou and colleagues sought to determine the etiological pathogens responsible for cases of CNIE treated at a tertiary care hospital in France using a molecular diagnostic test.

The investigators employed a highly sensitive universal microbial detection kit called the UMD-biomolecular test that uses 16S (for bacteria) and 18S (for fungi) ribosomal deoxyribonucleic acid (rDNA) gene amplification on excised cardiac samples (valves and vegetations) from patients who had undergone surgery for endocarditis, hereafter referred to as T0. Each kit contains positive and negative controls. Samples were collected between Jan. 1, 2011, and Dec. 31, 2015. Patients were considered to have CNIE when clinical and echocardiographic findings met the Duke criteria for definitive endocarditis without microbiological identification, which included 10 days of negative blood cultures, 14 days of negative cardiac tissue cultures, in house 16S-rDNA polymerase chain reaction (PCR), and negative serologies for Coxiella burnetii, Bartonella, and Brucella.

During the time frame for the study, there were 88 cases that met the Duke criteria for definitive endocarditis and had excised cardiac tissue. Of these, 16 were classified as CNIE. The tissues came from prosthetic mitral (n = 9) and aortic (n = 7) valves. The mean age of the patients was 58 years (range, 25-82), 10 were women, and six were men. All 16 samples were positive using the UMD-biomolecular test. Of these, 13 were positive for bacteria and three were positive for yeast. Eight of the samples for bacteria were polymicrobial and nine were positive for species of Streptococcus. One sample was positive for Enterococcus cecorum, a commensal in the intestines of mammals and birds that has been described as an emerging pathogen in the poultry industry worldwide, and one was positive for Moraxella spp.1 The other three samples were 16S-rDNA PCR negative but 18S-rDNA PCR positive and all identified as Malassezia restricta.

Histological analysis of the three cases of M. restricta found single rounded or oval yeast-form cells 2 µm to 3 µm in length in two samples and short unbranched mycelial pseudohyphae with budding yeast forms in one sample. Notably, sera from two of the three patients with M. restricta cross-reacted with C. albicans antibodies in the screening and confirmation assays.

All three of the patients had undergone cardiac valve replacement two to 15 months before T0. Patient 1 was a 66-year-old man who had a new heart valve placed at T0 and was treated with vancomycin, gentamicin, and rifampin. He never received antifungal treatment and died six years later with fever and heart failure. Patient 2 was a 72-year-old woman who underwent a second valve replacement at T0 and was treated with daptomycin, ceftriaxone, and gentamicin. Two weeks later, she became septic and was treated with caspofungin. She developed a subcutaneous nodule on her right flank that was positive for M. restricta by 18S-rDNA PCR. Two months later, a transesophageal echocardiogram revealed a peri-prosthetic valve abscess, and she died six months afterward. Patient 3 was a 42-year-old woman who also had another heart valve placed and was treated with daptomycin, ciprofloxacin, and imipenem. She developed a vegetation on the new heart valve three months later and was treated with amphotericin B and fluorocytosin. The vegetation cleared, and she was maintained on fluconazole thereafter. All three patients had either pruritic skin lesions or received lipid parenteral nutrition in the weeks to months prior to the onset of their symptoms.


This was an interesting study because it showed the usefulness of a commercial molecular test kit for identifying the etiologies of CNIE after traditional testing had led to a diagnostic dead end. It also was impressive that the kit was able to make the diagnosis, while the in-house 16S-rDNA assay was not. Perhaps this was because of more vigorous quality and manufacturing controls in the commercial assay. Also, there was no in-house 18S-rDNA PCR test available for fungal testing during the study time frame.

M. restricta is a lipophilic yeast and one of the most abundant Malassezia species of the human skin microbiota. It is considered an opportunistic pathogen associated with skin disorders, such as seborrheic dermatitis and dandruff.2 Although there have been case reports of infections caused by other types of Malassezia, including a case of endocarditis from M. furfur, this is the first report of endocarditis caused by M. restricta. Because of their inherently low pathogenicity, Malassezia easily can be considered colonizers or contaminants in clinical samples. Malassezia are difficult to grow with routine culture methods because they are unable to synthesize medium and long chain fatty acids, therefore requiring special culture media containing added lipids. The pathophysiology for Malassezia endocarditis remains unknown, although an obvious hypothesis is acquisition from the human skin and contamination of vascular catheters.

The cross-reactivity between M. restricta and C. albicans serology is concerning. Often cardiac tissue is not available in cases of CNIE, so 16S rDNA and 18S rDNA testing cannot be performed. Thus, clinicians must rely on fungal serologies to diagnose fungal endocarditis when blood cultures and antigen tests are negative. Malassezia is intrinsically resistant to echinocandins, which is the recommended treatment of endocarditis caused by Candida. Of the three patients with M. restricta endocarditis in the present study, only one received appropriate antifungal therapy.

Diagnosing endocarditis caused by Malassezia is challenging. Thus, it is worth mentioning the current recommendation that histological staining and molecular testing, including for fungi, should be performed on cardiac samples when the causative pathogen is obscure.


  1. Dolka B, Chrobak-Chmiel D, Czopowicz M, Szeleszczuk P. Characterization of pathogenic Enterococcus cecorum from different poultry groups: Broiler chickens, layers, turkeys, and waterfowl. PLoS One 2017;12:e0185199.
  2. Morand SC, Bertignac M, Iltis A, et al. Complete genome sequence of Malassezia restricta CBS 7877, an opportunist pathogen involved in dandruff and seborrheic dermatitis. Microbiol Resour Announc 2019;8:e01543-18.