Microbiology Case Study: A 54 Year Old Male with Shortness of Breath

Case History

A 54 year old male presented in the emergency room with worsening of shortness of breath and chest pain. He has a history of a bicuspid aortic valve that was treated with bio-prosthetic aortic valve replacement seventeen years ago and a second aortic valve replacement seven years ago. The patient’s echocardiogram showed severe aorticstenosis and moderate to severe mitral regurgitation. During hospital stay he started to show signs of low cardiac output syndrome and an intra-aortic balloon pump was placed. During sternotomy for aortic valve replacement and mitral valve repair they discovered a severely calcified and stenotic valve with additional debris that could be consistent with endocarditis. Tissue culture was sent.

Gram stain showed pink strings that could be gram negative rods, but could also be tissue debris due to tissue grinding (Image 1). After 3 days of incubation, some colonies grew on 5% sheep blood (Image 2) and chocolate agar plates with no growth on MacConkey selective medium.

Colony Gram stain made from these colonies (Image 3) was compared with the initial gram stain and showed similar type of pleomorphic gram negative rods. MALDI-TOF identified this organism as Cardiobacterium hominis.

Discussion

Cardiobacterium hominis is a fastidious, pleomorphic, non-motile, gram negative bacillus and member of the HACEK group which comprises Haemophilus species, Aggregatibacter, Cardiobacterium hominis, Eikenella corrodens, and Kingella kingae. C. hominis is present as normal flora of the oropharynx in most individuals but it has also been attributed to cause infective endocarditis.

C hominis is a fastidious bacterium that grows best in the presence of increased levels of CO₂ and high levels of humidity and often takes several days to grow on solid media (1). It can be distinguished from other HACEK members by a positive oxidase reaction, the production of indole and the absence of catalase activity and nitrate production.

Some of the risk factors leading to C hominis endocarditis include dental work, structural cardiac abnormalities, previous valve replacement, dilated cardiomyopathy and past history of rheumatic heart disease and endocarditis (2). The illness usually follows a sub acute course with symptoms lasting for weeks or months (1). Patients will often report fever, myalgia, anorexia, and weight loss. C. hominis tends to form large, friable vegetations associated with cerebral embolization or mycotic aneurysm formation and this might be responsible for atypical presentation of endocarditis(1). The overall prognosis of endocarditis due to C. hominis is quite favorable, despite the frequent need fo rvalve replacement (3).

Third generation cephalosporin (ceftriaxone) is considered the drug of choice for C. hominis endocarditis. Ampicillin can be used after susceptibility testing. Ampicillin-sulbactam or ciprofloxacin are alternative therapeutic options.

References

1. Currie, Codispoti, Mankad, et al. Late aortic homograft valve endocarditis caused by Cardiobacterium hominis: a case report and review of the literature. Heart 2000;83:579–581.
2. Walkty A. Cardiobacterium hominis endocarditis: A case report and review of the literature. The Canadian Journal of Infectious Diseases & Medical Microbiology. 2005;16(5):293-297.
3. Fazili T, Endy T, Javaid W, Amin M. Cardiobacterium Hominis Endocarditis of Bioprosthetic Pulmonic Valve: Case Report and Review of Literature. J Clin Case Rep. 2013;3:286.

-Kiran Manjee, MD, is a 1st year anatomic and clinical pathology resident at University of Chicago (NorthShore).

-Erin McElvania, PhD, D(ABMM), is the Director of Clinical Microbiology NorthShore University Health System in Evanston, Illinois. Follow Dr. McElvania on twitter @E-McElvania.