Post-treatment bacterial endocarditis mimicking fungal organisms: a morphologic comparison and tips for avoiding this diagnostic pitfall.

Background: Histopathologic differentiation of bacterial endocarditis from yeast-like fungal endocarditis is usually straightforward; however, an underappreciated phenomenon is the effect of antimicrobial therapy on bacterial size, shape and septa (cross-wall) formation resulting in bacterial forms that mimic yeast-like fungi. In this article we illustrate the alterations that occur in antibiotic-treated Staphylococcus aureus endocarditis and compare these changes to histopathologic findings in unaltered S. aureus and Histoplasma endocarditis, respectively.

Methods: Resected valves from three cases of endocarditis were compared based on the type ofinflammatory reaction, organism morphology and culture results. Case 1 was S. aureus endocarditis initially misclassified as Histoplasma due to its atypical morphologic and histopathologic features. The two cases included for comparison were an S. aureus endocarditis with more classic features and an Histoplasma capsulatum endocarditis. Hematoxylin and eosin (H&E), Gram, periodic acid Schiff (PAS), Gomori-Grocott methenamine silver stains (GMS), and culture results were compared in all cases. Molecular and immunohistochemistry tests were used for confirmation of first case. High power oil-immersion was used to visualize organisms' characteristics in all three cases.

Results: Case 1 and Case 3 (Histoplasma-infected valves) had fibrinous exudates with scattered macrophages. The microorganisms observed in the first case of methicillin-sensitive S. aureus (MSSA) were ∼ 2-3 μm by GMS stain and had prominent septations. Histoplasma yeast were round to oval, ∼ 3-4 μm in size and demonstrated budding. S. aureus without alterations were round, ∼ 1 μm in size, and lacked prominent septations. Necrotizing purulent inflammation was present in the unaltered case of MSSA. The MSSA case with alterations from antibiotic treatment did not stain well with the Gram stain and organisms were best visualized with the PAS and GMS stains.

Conclusions: Antibiotic therapy for bacterial endocarditis can alter the inflammatory reaction to infection, bacterial size, septa formation, and staining characteristics. Knowledge of these therapy-related effects and use of high-power magnification helps to avoid misclassification as yeast-like fungi.