FISH for Identification and Visualization of Microorganisms in Heart Valve Tissue Derived by Cardiac Biopsy in Culture-Negative IE—Is it Feasible?

Objectives: Identification of the causative organism in infective endocarditis (IE) can be difficult, especially in right-sided endocarditis, but it is crucial for patient treatment and survival. FISH (Fluoreszenz in situ Hybridisierung) is a molecular, culture-independent technique that allows to identify and visualize microorganisms within tissue and to recognize their morphology, number and activity. We analyzed the diagnostic benefit of FISH/PCR by comparing its results to those of standard diagnostic tests in two complex cases with IE.

Methods: In two patients, where balloon valvuloplasty was performed to treat severe pulmonary stenosis and signs of right ventricular failure, tissue specimens were obtained by cardiac biopsy because of clinical signs and suspicious ultrasound imaging for IE. FISH/PCR samples were fixed in FISH fixation solution in the cath laboratory and sent to the Biofilmcenter, Deutsches Herzzentrum, Berlin. In each case, additional blood culture were taken in cath laboratory from pulmonary artery. In total, more than 12 in each patient. Patient 1: 21-year-old female,. pulmonary atresia and VSD, and melody valve 2009. No improvement after 3 weeks of therapy. Blood cultures were sterile. Biopsy from Melody Pulmonary Valve. Detection of Streptococcus salivarius. Patient 2: 22–year-old male. Ross procedure 2015. Blood cultures were sterile. Biopsy from pulmonary homograft. Detection of Staphylococcus epidermidis.

Result: In two cases of culture negative IE, a single causative microorganisms could be identified; no mixed infection was detected by FISH/PCR. In our second case, epidermidis (part of regular skin flora) was detected and in blood culture contamination would be supposed. Thus, we were able to identify microorganisms in cases where standard diagnostic tests failed to provide sufficient results.

Conclusion: In complex, apparently negative cases where under antibiotic treatment, bacteria organized in biofilms can be metabolically less active but viable. These populations are often missed in culture. However, an approach combining histopathological and molecular techniques (FISH/PCR) may lead to a definite final diagnosis which may guide targeted antibiotic therapy.