Bacteria

Morphology

Cytology:

• Basophilic filamentous, branching gram-positive colonies with an eosinophilic splendore-hoeppli rim (“sulfur granules”)
• Central dense core with radiating fibres that branch at acute angles
• Central dense core stains dark purple or black on Papanicoloau stain
• Background often neutrophil-rich +- multinucleated giant cells
• AFS (ZN) negative

Histology:

• Compact bacterial colonies with eosinophilic club-like material
• AFS (ZN) negative
• Highlighted by Gram stain, PAS, and GMS/Grocott
• Surrounding fibrosis
• Mixed suppurative inflammation

Differential diagnosis:

•  Nocardia spp. - partially acid-fast aerobes; Actinomyces are non-acid-fast anaerobes
• Pseudoactinomycotic radiate granules - particularly in IUD-associated specimens: broad irregular eosinophilic clubs without a central filamentous bacterial core; Gram and GMS stains are negative or non-specific. (1)

Clinical notes

Chronic, indurated masses with draining sinus tracts (“lumpy jaw”); thoracic disease can mimic malignancy; abdominopelvic forms after perforation or IUD. Generally good but slow response to prolonged high-dose penicillin therapy; additional drainage or surgical debridement may be required. (2)

Asymptomatic individuals with Actinomyces-like organisms on cervical cytology are more likely to be colonised than infected; neither antibiotic treatment nor IUD removal is required. (FSRH Guideline: Intrauterine Contraception. March 2023)

Ancillary Testing

Anaerobic culture with prolonged incubation; MALDI-TOF; 16S rRNA sequencing if needed. Consider broad PCR from tissue in culture-negative cases. Colonies: small, rough, heaped “molar-tooth” colonies on anaerobic blood agar, firmly adherent, white to gray-yellow and often chalky, with spidery/rhizoid edges after prolonged anaerobic incubation.

Additional Information

Detection in non-sterile mucosal samples may represent colonisation (oral/GI/genital tract commensal); tissue invasion, sulfur granules and associated suppurative inflammation support true infection.