Nocardia

Nocardia is a genus of aerobic, Gram-positive, branching filamentous bacteria in the family Nocardiaceae, order Actinomycetales, within the phylum Actinobacteria. They are partly acid-fast due to mycolic acids in their cell walls. Nocardia species are commonly found in soil, decaying vegetation, and water, and infections are typically acquired through inhalation or direct inoculation into the skin. Human-to-human transmission is rare.

Nocardia causes nocardiosis, an opportunistic infection that often affects immunocompromised individuals, including those with HIV/AIDS, organ transplants, or chronic corticosteroid use. The lungs are the most frequent primary site, presenting as pneumonia or bronchitis that can resemble tuberculosis or fungal infections. Extrapulmonary disease can involve the brain, producing abscesses, or the skin and soft tissues, causing nodules or cellulitis. Dissemination to the central nervous system is a serious complication.

Diagnosis relies on microbiological culture and microscopy. Specimens from affected sites are cultured on standard media; Nocardia grows slowly and may appear chalky, dry, or powdery with aerial hyphae. A modified acid-fast stain can reveal partially acid-fast, branching filamentous rods. Species identification is increasingly aided by molecular methods (such as 16S rRNA sequencing) or MALDI-TOF mass spectrometry, which can influence treatment choices.

Treatment commonly starts with trimethoprim-sulfamethoxazole, often in combination with another agent such as imipenem, amikacin, or linezolid for severe disease or CNS involvement. Therapy is typically prolonged, lasting several months to a year, depending on site and immune status, and susceptibility testing is advised due to variable resistance patterns.

Epidemiology is worldwide, with Nocardia species isolated from soil and environmental reservoirs. The organism is not considered contagious between people. Prevention focuses on reducing exposure for high-risk individuals; there is no vaccine.