Nocardia

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The page below is a sample from the LabCE course Medically Important Aerobic Actinomycetes. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Nocardia

The Nocardia genus contains over 100 species; roughly half are associated with humans. Clinically, Nocardia causes mycetoma, lymphocutaneous infections, skin abscesses, cellulitis, invasive pulmonary infections, and/or disseminated infections (ie, 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 nomenclature38 has been updated to include the following major changes:
  • 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 Note, however, the term, ‘N. asteroides group’ is now outdated and should be avoided.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)

CharacteristicsComments
Appearance on Gram stainGram-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 disease39
Nocardia nova, N. farcinica, N. cyriacigeorgica, N. brasiliensis, N. abscessus
Colonial growthCreamy or chalky, matte or velvety, wrinkled/dry, pigmented (orange, pink, brown, red, purple, gray, white, tan), adhere firmly to agar; some aerial growth with cotton wool ball appearance.40 Note: the presence of aerial hyphae differentiates Nocardia from Rhodococcus, Gordonia, and Tsukamurella.41
Phenotypic characterization40,41
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 growthGrowth at 45°C after 3 days (variable and dependent on species)
38. Mazumder SA. Nocardiosis workup. March 9, 2021. Accessed December 22, 2022. https://emedicine.medscape.com/article/224123-workup
39. CDC. Nocardiosis information for healthcare workers. Website reviewed March 2016. Accessed November 1, 2022. https://www.cdc.gov/nocardiosis/health-care-workers/index.html
40. Tankeshwar A. Nocardia: Properties, pathogenesis, lab diagnosis. Updated August 1, 2022. Accessed November 5, 2022. https://microbeonline.com/nocardia-properties-pathogenesis-diagnosis/
41. National Health Service. Public Health England: UK standards for microbiology investigations - Identification of the aerobic actinomycetes. 2016. Accessed November 5, 2022. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/563971/ID_10i2.2.pdf
42. CDC. Nocardiosis information for healthcare workers: Modified Kinyoun's acid-fast stain of Nocardia cyriacigeorgica grown on sheep blood agar 5 days at 35°C, 100x. Public domain image. Website reviewed March 2016. Accessed November 1, 2022. https://www.cdc.gov/nocardiosis/health-care-workers/index.html
43. CDC/Georg. 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. Created 1964. Accessed October 27, 2022. https://phil.cdc.gov/Details.aspx?pid=22297

42. Nocardia cyriacigeorgica seen in modified Kinyoun acid-fast stain of sheep blood agar.
43. Nocardia brasiliensis on agar