All three organisms are commonly found in soil. In addition to being gram positive, these three species are modified acid-fast positive. This characteristic helps differentiate Nocardia species from the Actinomyces organism , a gram-positive, anaerobic pathogen that also forms branching filaments. The Nocardia organism grows slowly on blood agar plates and Sabouraud's dextrose agar. In sputum specimens, other organisms often overgrow, obscuring Nocardia colonies.
Colonies generally have an orange pigment and appear heaped up and folded.
They can also be white or pink. Colony characteristics suggestive of the Nocardia organism may not develop for two to four weeks. Laboratory staff need to be alerted to incubate cultures for a prolonged period when a Nocardia species is suspected. Samples should be obtained when the patient is not taking antibiotics. In most cases, the Nocardia organism gains entry to the host through the respiratory tract.