September 2021 – Clinical Microbiology Case 1

A 64-year-old man involved in a minor MVA was found to have a new parietal brain lesion and 5 mm pulmonary nodule on CT. He reported two months of minor upper respiratory symptoms and a two-week history of headaches with associated cognitive difficulties, weakness, and fine motor deficits. MRI of the brain showed a 2.3 cm cystic “ring enhancing” lesion (Fig. 1). Intraoperative evaluation of the brain biopsy showed fibrinous material with acute inflammation. Organism morphology on GMS (Fig. 2), modified acid-fast stain (Fig. 3), and culture medium (Fig. 4) are shown. 

Figure 1: MRI of the brain showing lesion
Figure 2: GMS
Figure 3: Modified acid-fast stain
Figure 4: Culture medium

Which of the following is true regarding this organism?

  • Calcium phosphate binds this organism together to form “sulfur granules.”
  • Colonies turn yellow after exposure to light (photochromogen).
  • It is acquired through contact with contaminated soil.
  • It is a “beaded” filamentous gram-negative bacillus.

The correct answer is ...

The correct answer is: It is acquired through contact with contaminated soil.

This is a case of Nocardiosis with pulmonary and CNS involvement. The ring enhancing lesion on MRI corresponds to the dense fibrous capsule surrounding the abscess cavity. GMS reveals clusters of filamentous organisms that display a characteristic “beaded” morphology on a modified acid-fast stain. Colonies are peach to chalky-white and grow in 3 to 5 days on Sabouraud medium or blood agar. MALDI-TOF mass spectrometry performed on positive broth culture identified Nocardia transvalensis. Due to variable resistance patterns, cultures and sensitivity testing should be performed to guide antibiotic therapy. While CNS treatment guidelines are initial drainage followed by antibiotic therapy for 12 months, recurrence after discontinuing treatment is common. Despite aggressive treatment, CNS Nocardiosis mortality rates remain high: 20% in immunocompetent patients and 55% in immunocompromised patients.

There are over 100 species of Nocardia, with at least 15 species known to infect humans, most often occurring in an immunocompromised host. They are found in soil and rotting vegetation and inhalation of contaminated dust is the most common route of exposure. On Gram stain, Nocardia spp. are “beaded” filamentous, and branching, gram-positive organisms and are partially acid-fast, differentiating them from Actinomyces spp., which are not acid-fast and characteristically produce “sulfur granules” in tissue. Colonies of Nocardia grown in culture do not require exposure to light to display the characteristic soft peach color, as opposed to photochromogenic species of mycobacteria, such as M. kansasii.


  1. Goldman, L., Schafer, A. I., & Cecil, R. L. (2020). Chapter 314: Nocardiosis. In Goldman-Cecil medicine (26th ed., pp. 2032–2034). essay, Elsevier. 
  2. Sun, H., Goolam Mahomed, M., & Patel, J. (2021). Brain metastasis or nocardiosis? A case report of central nervous system Nocardiosis with a review of the literature. Journal of community hospital internal medicine perspectives11(2), 258–262.

Holly Berg, D.O., MLS(CM)

Resident, Anatomic and Clinical Pathology
Mayo Clinic

Nancy Wengenack, Ph.D.

Consultant, Clinical Microbiology
Mayo Clinic
Assistant Professor of Laboratory Medicine and Pathology
Mayo Clinic College of Medicine and Science

MCL Education (@mmledu)

MCL Education

This post was developed by our Education and Technical Publications Team.