Infection with filamentous rods—suspect Actinomyces species infection is correct!
These organisms are much more delicate than fungal hyphae and lack an obvious cell wall. The primary differentials for filamentous rods are Actinomyces, Nocardia, and Fusobacterium species infections. These organisms can be found in the environment (including soil) and can cause infection after a penetrating wound or migration of some type of plant fiber. Actinomyces species are also part of normal oral flora. Clumps of Actinomyces or Nocardia species embedded in a proteinaceous matrix, when floating in thoracic effusions, often appear pale yellow, leading to the term sulfur granules. Both Actinomyces and Nocardia species are weakly gram-positive, while Fusobacterium species are gram-negative. Nocardia species are also weakly acid-fast-positive. Actinomyces and Fusobacterium species are anaerobic, while Nocardia species are a slow-growing aerobe.
In this case, anaerobic culture results were positive for Actinomyces species, and infection was probably initiated by a bite wound.