Diagnostic and management challenges in dual-pathogen culture-negative endocarditis: a case report of Brucella Canis native valve infective endocarditis and subsequent saccharomyces cerevisiae prosthetic valve infective endocarditis.

BACKGROUND: Infective endocarditis caused by fastidious organisms, such as rough Brucella canis, can evade standard serology, risking unrecognised native-valve destruction and secondary prosthetic valve endocarditis. CASE PRESENTATION: A woman in her 40 s developed native aortic IE requiring composite mechanical root replacement; cultures were sterile. Nine months post-operatively, she presented with mechanical prosthetic valve endocarditis and paravalvular regurgitation, undergoing redo homograft replacement. Retrospective rough-lipopolysaccharide ELISA of archived serum confirmed persistent B. canis seropositivity. Multi-target PCR across two independent reference laboratories identified superimposed Saccharomyces cerevisiae prosthetic valve endocarditis, corroborated by histology. MANAGEMENT & OUTCOME: Doxycycline-induced gastritis and drug interactions necessitated modifying antibacterial therapy to amikacin, co-trimoxazole, and rifampicin; fungal endocarditis was treated with liposomal amphotericin B and subsequent posaconazole suppression. At six months, transoesophageal echocardiography and [¹⁸F]-FDG PET-CT confirmed complete infection resolution. CONCLUSION: This case highlights suspecting rough Brucella in culture-negative infective endocarditis, evaluating unexpected molecular fungal detections alongside histology, and tailoring regimens around severe antimicrobial toxicities.