The Nocardia genus contains over 100 species, roughly half of which are associated with humans. Clinically, Nocardia causes mycetoma, lymphocutaneous infections, skin abscesses, cellulitis, invasive pulmonary infections, and/or disseminated infections (e.g., brain).
In the past, suspected Nocardia isolates were identified using microscopy and hydrolysis of casein, tyrosine, xanthine, and hypoxanthine. Now, nucleic acid and proteomic characterization have assisted in the updating of taxonomy. Nocardia nomenclature has been updated to include the following major changes:23
- Once known as N. asteroides: Now, the N. asteroides group, consisting of N. abscessus, N. nova complex, N. farcinica, N. brevicatenapaucivorans complex, N. cyriacigeorgica, and others. Note, however, the term ‘N. asteroides group’ is now outdated and should be avoided. The individual species stand on their own.
- Once known as N. brasiliensis: Now known as N. brasiliensis (primarily skin) and N. pseudobrasiliensis (primarily pulmonary). (The other Nocardias cause pulmonary and disseminated, but not so much primary skin infections.)
- Newly recognized species: N. africana, N. paucivorans, N. veterana (not much is known yet about these)
Table 1. Characteristics of Nocardia species.| Characteristics | Comments |
| Appearance on Gram stain | Gram positive, filamentous, branched filaments 0.5–1.2 microns in diameter; beaded appearance |
| Appearance on modified acid-fast stain (MAS) | Partially/variably acid-fast (red color) |
| Most often associated with clinical disease24 | Nocardia nova, N. farcinica, N. cyriacigeorgica, N. brasiliensis, N. abscessus |
| Colonial growth | Creamy or chalky, matte or velvety, wrinkled/dry, pigmented (orange, pink, brown, red, purple, gray, white, tan), adheres firmly to agar; some aerial growth with cotton wool ball appearance.25 Note: The presence of aerial hyphae differentiates Nocardia from Rhodococcus, Gordonia, and Tsukamurella.26 |
| Phenotypic characterization25,26 | - Casein, xanthine, tyrosine hydrolysis: species-dependent
- Acetamide utilization
- Gelatin hydrolysis positive
- Lysozyme resistance positive
- Urea hydrolysis positive
- Nitrate reduction variable (most isolates are positive)
- Acid from rhamnose
|
| Temperature of optimal growth | Growth at 45°C after 3 days (variable and dependent on species) |
23. Mazumder, S. (2024). Nocardiosis. Medscape. https://emedicine.medscape.com/article/224123-overview#a424. Clinical overview of nocardiosis. (2025). Centers for Disease Control and Prevention. https://www.cdc.gov/nocardiosis/hcp/clinical-overview/?CDC_AAref_Val=https://www.cdc.gov/nocardiosis/health-care-workers/index.html
25. Tankeshwar, A. (2024). Nocardia: Properties, pathogenesis, lab diagnosis. Microbe Online. https://microbeonline.com/nocardia-properties-pathogenesis-diagnosis/
26. National Health Service. (2016). UK standards for microbiology investigations: Identification of aerobic actinomycetes. The Royal College of Pathologists. https://www.rcpath.org/static/ce08d742-b58e-4e8d-986c43a75ac367c3/uk-smi-id-10i2-2-identification-of-aerobic-actinomycetes-october-2016-pdf.pdf
Figure 17. CDC/Georg. (1964). Image #22297. This Petri dish culture plate contained an unidentified growth medium, which had been inoculated with the bacterium Nocardia brasiliensis, the cause of the disease, nocardiosis. After undergoing a 3-week incubation period, the culture gave rise to this wrinkled, roughly textured, beige-colored colony. PHIL public domain. https://phil.cdc.gov/Details.aspx?pid=22297
