acid-fast bacilli

**Basis of acid-fastness:** The mycobacterial cell wall is rich in mycolic acids (long-chain, waxy fatty acids), along with arabinogalactan and lipoarabinomannan. This makes the wall very lipid-rich and hydrophobic, so Gram stain works poorly (organisms appear "ghost-like" or weakly Gram-positive/beaded). Once carbol fuchsin (a red dye) is driven into the wall with heat (Ziehl-Neelsen) or high phenol concentration (Kinyoun, "cold" method), the organisms retain it despite washing with acid-alcohol. Non-acid-fast cells and background are decolorized and take up the counterstain (methylene blue or malachite green).

**Staining methods:** - Ziehl-Neelsen (hot): carbol fuchsin + heat, acid-alcohol decolorizer, methylene blue counterstain. AFB appear red on a blue background. - Kinyoun (cold): higher phenol concentration, no heat. - Auramine-rhodamine (fluorescent): more sensitive, used for screening. AFB fluoresce yellow-green against a dark background.

**Organisms:** - *Mycobacterium tuberculosis*: strongly acid-fast. Slow-growing obligate aerobe, with cord factor (trehalose dimycolate) as a virulence factor. Grows on Löwenstein-Jensen medium, Middlebrook agar, or in liquid culture. Nucleic acid amplification tests (e.g., Xpert MTB/RIF) give rapid detection and rifampin-resistance screening. - *M. leprae*: acid-fast, cannot be cultured in vitro (grown in armadillo footpads or mouse footpads). Prefers cooler body sites (skin, peripheral nerves). Tuberculoid vs. lepromatous forms. - *M. avium-intracellulare complex (MAC)*: disseminated disease in advanced HIV (CD4 <50/mm³). Prophylaxis with azithromycin. - *M. kansasii*: TB-like pulmonary disease, especially in COPD. *M. marinum*: "fish tank granuloma" on the skin. *M. scrofulaceum*: cervical lymphadenitis in children. - **Partially (weakly) acid-fast:** *Nocardia* (aerobic, branching filamentous, Gram-positive beaded rods; weakly acid-fast with modified Kinyoun using 1% sulfuric acid; causes pulmonary and brain abscesses in immunocompromised patients and is treated with TMP-SMX), *Rhodococcus equi*, and *Gordonia*. - **Acid-fast protozoa:** oocysts of *Cryptosporidium* (watery diarrhea in AIDS patients; treated with nitazoxanide), *Cyclospora cayetanensis* (raspberries/imported produce; treated with TMP-SMX), and *Cystoisospora belli*. These use modified acid-fast stain. - Bacterial endospores and *Taenia* eggs are not routinely considered in this context, but note that spores of Bacillus/Clostridium are stained with malachite green, not acid-fast stains.

**Clinical significance:** - Sputum AFB smear: typically 3 early-morning samples. A positive smear indicates a high bacillary load and infectivity, but it is not species-specific, so it cannot distinguish M. tuberculosis from nontuberculous mycobacteria (NTM). Sensitivity is limited (about 50–70%), so culture and NAAT are used for confirmation. Culture remains the gold standard and allows drug susceptibility testing. - A negative smear does not exclude TB. - Respiratory isolation (airborne precautions, negative-pressure room, N95) is used for suspected pulmonary TB. - Histology: caseating granulomas with Langhans giant cells (horseshoe-shaped nuclei) and AFB on special stain. Leprosy shows AFB within foamy macrophages (lepromatous) or sparse AFB (tuberculoid). - Treatment of TB: RIPE (rifampin, isoniazid, pyrazinamide, ethambutol) for 2 months, then rifampin and isoniazid for 4 months. Pyridoxine (B6) is given with isoniazid to prevent neuropathy.

**High-yield pearls:** - "Acid-fast" = mycolic acids = waxy wall. Mycobacteria do not stain well with Gram stain. - Nocardia is *weakly* acid-fast (modified), and is a Gram-positive branching rod. Compare with Actinomyces, which is *not* acid-fast, is anaerobic, and forms sulfur granules (treated with penicillin). - Cryptosporidium oocysts are acid-fast in stool in AIDS patients with chronic watery diarrhea. - Interferon-gamma release assays and the tuberculin skin test detect immune sensitization, not the organism itself, and do not distinguish latent from active disease.