DISCUSSION:
Mycobacterium kansasii, a nontuberculous mycobacterium, was an unusual pathogen before the AIDS epidemic. Subsequent to the AIDS epidemic, Mycobacterium kansasii has become a more common pathogen causing respiratory infections as well as disseminated disease in patients with AIDS (1). Mycobacterium kansasii infection has found to be more related to the level of immunosuppression in AIDS patients that Mycobacterium tuberculosis infection (2). The radiologic findings in thoracic disease caused by Mycobacterium kansasii are similar to those in Mycobacterium tuberculosis. Mycobacterium kansasii infection has also been reported in patients with an underlying malignancy, usually hematologic, including leukemia (3,4).
Direct examination of AFB stained preparations reveal that the organisms are acid-fast beaded rods. The organism grows on Lowenstein-Jensen media, usually within several weeks depending on initial inoculum. If the colonies are not exposed to light they are cream colored, but if exposed to light they develop a yellow pigment. Organisms with this capability are known as photochromogens. The main differential diagnosis for the microbiology laboratory is to distinguish Mycobacterium kansasii from the other photochromogenic mycobacteria (M. asiaticum, M. marinum, and M. simiae) as well as the scotochromogen Mycobacterium gordonae. Mycobacterium gordonii is a rapid growing mycobacterium that produces a yellow colony in the dark as well as after exposure to light (scotochromogen). The most rapid differentiation of these organisms can be performed using DNA probe technology. In this case the M. gordonae probe was negative and the M. kansasii probe was positive.
This patient was reportedly HIV negative. No malignancy was ever definitively identified. It is very rare for an immunocompetent patient to develop disseminated Mycobacterium kansasii disease. The patient expired shortly after admission to the hospital. No autopsy was performed, so the any insight into any possible underlying immunocompromise will remain elusive.
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Contributed by Joel F Gradowski, MD and William Pasculle, ScD.
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