A 40-year-old man who has tested positive for human immunodeficiency virus (HIV) infection and who is undergoing highly active antiretroviral therapy (HAART) presents with progressive encephalomyeloradiculopathy. He has severe headaches but no fever, cough, or weakness. Cerebrospinal fluid (CSF) is collected. The test results for the specimen are: 25 WBC/mm3 (25 white blood cells per cubic millimeter), low glucose, elevated protein, and no organisms on Gram stain or acid-fast stain. His studies are negative for cryptococcal antigen, Toxoplasma organisms (by serology), and herpes simplex virus (HSV) (by polymerase chain reaction [PCR]). Routine bacterial culture is negative. Despite therapy for HSV and routine aerobic bacterial causes of meningitis, over the next 4 days the patient spikes fevers. A second CSF specimen shows 415 WBC/mm3, with no diagnosis. A battery of viral encephalitis serology tests are done, and all are negative. In-house PCR testing on a third CSF specimen is positive for Mycobacterium tuberculosis, which grows in culture after 4 weeks.

QUESTIONS

1. Why are the acid-fast smear results from all three of the specimens negative, but the second PCR result is positive?

2. How can M. tuberculosis be identified to the species level?

3. List the organisms present in the Mycobacterium tuberculosis complex.

4. Sometimes in processing for mycobacterial culture, an aerosol is created and one specimen splashes into another tube and contaminates it. If the physician states that the patient does not appear to have tuberculosis, how can the laboratory confirm that the positive culture does not represent contamination?