Mycobacteria

Mycobacteria are a genus of aerobic, non-motile bacilli defined by their unusual cell wall, which is packed with long-chain mycolic acids. This waxy, lipid-rich coat resists decolorization by acid-alcohol, so mycobacteria are identified with acid-fast stains such as Ziehl-Neelsen (appearing red) rather than the Gram stain, which penetrates the wall poorly.

The same cell wall explains much of the group's behavior. It slows nutrient entry, making most mycobacteria notoriously slow-growing; shields the organism from many disinfectants and antibiotics; and helps the bacteria survive inside macrophages, where they provoke granulomatous inflammation. Clinically important members include Mycobacterium tuberculosis (tuberculosis), M. leprae (leprosy), and nontuberculous species such as M. avium complex, an opportunistic pathogen in advanced HIV disease.

Tuberculosis screening is where most healthcare workers meet mycobacteria in practice. The tuberculin skin test (PPD) is an intradermal injection read 48–72 hours later for induration, reflecting delayed-type hypersensitivity to mycobacterial antigens; interferon-gamma release assays are the blood-based alternative. Because the organisms resist standard therapy, treatment of active tuberculosis requires prolonged multi-drug regimens.

Mycobacteria are core microbiology on the USMLE Step 1 exam, which tests acid-fast staining, cell wall structure, intracellular survival, granuloma formation, and the major diseases of each species. The CCMA exam covers the practical side — administering intradermal injections and performing tuberculin skin testing.