Bacteria

Morphology

Cytology:

• filamentous, branching at right angles, beaded rods
• weakly Gram-positive to Gram-variable
• partially acid-fast on modified acid-fast stains
• GMS positive
• acute inflammation and necrosis may be seen in background

Differential Diagnosis:

• Actinomyces spp. - non-acid-fast, anaerobic
• Mycobacteria spp. - non-branching
• Streptomyces spp. - non-acid-fast
• fungal mimics - Nocardia exhibits a branching morphology and shows positive staining with GMS; the presence of thin, delicate, beaded filaments can aid in distinguishing it from true fungi

Clinical notes

Opportunistic infection (lung first) in immunocompromised patients; imaging may show nodules, consolidations, cavitary lesions, or abscesses.; hematogenous spread to CNS (brain abscess) and skin/soft tissue after trauma, also in immunocompetent individuals. Potentially high mortality, particularly in disseminated disease, CNS involvement, immunosuppression, or delayed treatment. 

Ancillary Testing

Modified acid-fast (1% H₂SO₄); culture with prolonged incubation; MALDI-TOF; 16S rRNA/PCR; AST. Bacterial culture: chalky white to orange; wrinkled, dry to waxy, velvety on blood agar (1-3 days). On Löwenstein–Jensen (~10 days): moist, smoother colonies.

Additional Information

Correlation of cytological findings with culture and/or PCR recommended. In pulmonary nocardiosis, CNS imaging may be considered to exclude disseminated disease.