Nocardia

Nocardia is a genus of gram-positive bacteria that grow as long, thin, branching filaments, giving them a fungus-like appearance under the microscope. Two properties define the organism on exams: it is a strict aerobe, and it is weakly (partially) acid-fast, because its cell wall contains mycolic acids similar to those of mycobacteria. Nocardia lives in soil, and infection typically follows inhalation of the organism or direct inoculation through the skin.

The classic comparison is with Actinomyces, another gram-positive branching filamentous rod. The two are distinguished by oxygen requirement and acid-fast staining: Nocardia is aerobic and partially acid-fast, while Actinomyces is anaerobic, not acid-fast, and forms yellow "sulfur granules." A common mnemonic — SNAP — captures the pairing: Sulfa for Nocardia, Actinomyces gets Penicillin.

Clinically, Nocardia most often causes pulmonary nocardiosis, a pneumonia that can cavitate and mimic tuberculosis or lung cancer. It has a strong tendency to disseminate to the brain, forming abscesses, and it can produce cutaneous disease after soil-contaminated wounds. Most invasive infections occur in immunocompromised hosts — patients on chronic corticosteroids, transplant recipients, or those with impaired cell-mediated immunity — although cutaneous disease can occur in the immunocompetent. Sulfonamides such as trimethoprim-sulfamethoxazole are the standard treatment.

The USMLE Step 1 exam tests Nocardia in microbiology, most often as a compare-and-contrast with Actinomyces. Know the aerobic, partially acid-fast, branching morphology; the pneumonia-plus-brain-abscess pattern in an immunocompromised patient; and the sulfonamide association.