Answer: Actinomyces

Histology: The lesion is essentially an organizing abscess in which there are clumps of blue bacteria coated by pink immunoglobulin protein. This pattern is called the Splendore-Hoeppli pattern. The gram stain (Gram Weigert) and silver stain (GMS) demonstrate thin branching filamentous bacteria. A modified acid-fast stain was negative.

Discussion: Nocardia will typically yield identical findings to Actinomyces on gram and silver stains. Both are thin branching filamentous bacteria. However, Nocardia is classically positive with the modified acid-fast stain (Fite stain), while Actinomyces is not. One must remember that Nocardia may not always be positive with this stain so culture remains the gold standard for rendering these diagnoses. The thin 1 micron diameter filaments of Actinomyces and Nocardia are thinner than the two to six micron diameter filaments of Aspergillus. Bacterial pseudomycosis is caused by bacterial cocci like Staphylococcus aureus. This may also be associated with the Splendore-Hoeppli phenomenon on routine H& E stains, but the presence of cocci (as opposed to filamentous bacteria) is easily demonstrated on gram stains.

Actinomyces are part of the normal flora of the human mucosa, but tends to cause disease when mucosal barriers are breached. Hence, patients with periodontal disease are predisposed to cervico-facial actinomycosis, patients who aspirate (as the current patient did) are susceptible to thoracic actinomycosis, those with intestinal perforation are susceptible to abdominal actinomyces, and those women who use IUDs are susceptible to pelvic actinomyces. Treatment is with antibiotics (penicillin).