The genus Mycobacterium contains several species of bacteria that are pathogenic to humans (Table 7.2). For example, M. tuberculosis is spread from person to person through inhalation of airborne respiratory secretions containing mycobacteria expelled during coughing, sneezing, or talking. In patients with HIV/AIDS, Mycobacterium avium–intracellulare (MAI) complex is acquired through the GI tract, often through ingestion of contaminated water or food.
The disease progression of mycobacteriosis, particularly in patients with HIV/AIDS, is rapid (a few weeks). This short time span has required new methods for rapid recovery and identification of mycobacteria so that antibiotic therapy can be instituted promptly. These techniques involve the use of instruments that shorten the growth period for mycobacteria to 1–2 weeks. Isotopic nucleic acid probes are available for culture identification of M. tuberculosis, MAI complex, Mycobacterium kansasii, and Mycobacterium gordonae. Nucleic acid amplification assays, which use DNA technology to detect mycobacteria directly in clinical specimens, are also available to clinical laboratories.
Multidrug-resistant TB (MDR TB) describes TB that is resistant to the two most powerful anti-TB drugs: isoniazid and rifampin. Extensively drug-resistant TB is a rare subset of MDR TB, which is especially concerning to persons with HIV or severely depressed immune systems. Since 2012, MDR TB detection and diagnosis is increasing with the use of Xpert MTB/RIF assay (Cepheid, Sunnyvale, CA) in 77 countries.
Sputum and bronchial aspirates and lavages are the best samples for diagnosis of pulmonary infection. Purulent sputum (5–10 mL) from the first productive cough of the morning should be expectorated into a sterile container. If the specimen is not processed immediately, it should be refrigerated. Pooled specimens collected over several hours are not acceptable. For best results, three specimens should be collected over several days. A prerequisite of good specimen collection is the use of sterile, sturdy, leak-proof containers placed into biohazard bags.
AFB smears and cultures are done to determine whether TB-like symptoms are due to M. tuberculosis infection or infection from another mycobacterium and to aid in determining whether TB is intrapulmonary or extrapulmonary.
If the patient is unable to produce sputum, an early-morning gastric sample may be aspirated and cultured. This specimen must be hand delivered to the laboratory to be processed or neutralized immediately.
Patients with suspected kidney disease should provide early-morning urine specimens collected for 3 days in a row. Pooled 24-hour urine collections are not recommended. Unless processed immediately, the specimen should be refrigerated.
If TB meningitis is suspected, at least 10 mL (2 mL in children) of CSF should be obtained.
Sterile body fluids, tissue biopsy samples, and material aspirated from skin lesions are acceptable specimens for mycobacterial cultures. Tissue should be placed in a neutral transport medium to avoid desiccation. Swab specimens are not suitable for mycobacterial culture.
Feces are commonly the first specimens from which MAI complex can be isolated in a patient with disseminated disease. An acid-fast stain can be performed directly.
MAI complex organisms can also be isolated from the blood of immunosuppressed patients.
Procedural Alert
Acid-fast organisms stain red; however, not all mycobacteria take up the stain, and cultures may need to be done.