Answer to March 2022 Photo Quiz

Ghost cells of Mycobacterium spp. in the stool. Ghost cells were visualized on the microsporidia modified trichrome stain (Fig. 1A in the Photo Quiz), a phenomenon that results when Mycobacterium spp. resist stain uptake due to the high lipid content present in the cell wall. Observations from the modified acid-fast stain (Fig. 1B) further confirmed the presence of many positive acid-fast bacilli (AFB) in the stool. In contrast, no ghost cells were observed by the Wheatley trichrome method for intestinal amebae and flagellates (data not shown). Because the observation of AFB on stool smears is uncommon, these findings prompted the clinical team to submit a fresh stool specimen for AFB culture and further identification. The fluorochrome stain direct from the specimen demonstrated 3+ AFB. On day 6 of culture incubation, heavy growth (4+ AFB) of Kinyoun positive bacteria was observed from Middlebrook 7H10, Mitchison, and the Mycobacterium Growth Indicator Tube. The organism was identified as Mycobacteroides (Mycobacterium) chelonae by matrix-assisted laser desorption ionization-time of flight mass spectrometry.

Reports of nontuberculous mycobacteria (NTM) disease have increased in hematopoietic stem cell transplant and solid organ transplant recipients (1). Sites of infection vary from pleuropulmonary disease to skin and soft tissue, intraabdominal, and disseminated disease; however, intestinal NTM infection remains quite rare (2, 3). In this patient, following identification of these positive stool studies, a colonoscopy was undertaken, and histopathology from nodules in the sigmoid colon identified the presence of rare AFB positive organisms on a background of a dense lymphocytic infiltrate, the latter of which was consistent with intestinal posttransplant lymphoproliferative disorder. In addition, mycobacterial blood and respiratory cultures were collected to assess for disseminated disease; however, both were negative. Given concern for NTM colitis, the patient was treated with tobramycin, moxifloxacin, and azithromycin.

Stool is not routinely submitted for mycobacterial culture; this test is generally reserved for patients with AIDS suspected to have disseminated Mycobacterium avium complex (MAC) infection (4). Previous recommendations proposed stool be cultured for mycobacteria only if the direct smear of unprocessed stool is positive for AFB (5). However, due to limited sensitivity (32% to 34%) (6), screening smears may not be effective for identifying patients at risk of an invasive gastrointestinal NTM infection. Current practice guidance recommends emulsifying ∼1 g of stool in saline and allowing gravitational separation of fecal particles before further processing the supernatant to improve smear and culture sensitivity (4). The prevalence and clinical significance of NTMs in stool specimens remain relatively unknown in healthy and immunocompromised patients. One US study reported an 18% stool AFB culture positivity rate from human immunodeficiency virus-infected patients (n = 2,176), with 90% of isolates identified as MAC (6).

In our case, special parasite stains led to the unexpected finding of AFB-positive organisms that were subsequently identified on culture to be M. chelonae resulting in an uncommon diagnosis of M. chelonae colitis. Therefore, we highlight the importance of recognizing the presence of ghost cells and/or poorly staining organisms, as their identification may lead to a critical diagnosis of an unexpected mycobacterial infection.