Acid Fast Suspectibility - 5 drugs

Treatment for acid‑fast organisms varies. For Mycobacterium tuberculosis: isoniazid, rifampicin, pyrazinamide, ethambutol (first‑line). For Mycobacterium leprae: dapsone, rifampicin, clofazimine. Atypical mycobacteria often need macrolides (clarithromycin, azithromycin), amikacin, rifamycins, ethambutol, fluoroquinolones or linezolid. Nocardia (weakly acid‑fast) is treated with trimethoprim‑sulfamethoxazole, sometimes imipenem or amikacin. Choice depends on species, susceptibility testing and clinical context; combinations prevent resistance and require specialist guidance.

The four commonly cited acid‑fast organisms are Mycobacterium species (e.g., M. tuberculosis, M. leprae), Nocardia species (weakly acid‑fast), Cryptosporidium spp. (acid‑fast oocysts in stool), and Cyclospora cayetanensis (variable/modified acid‑fast oocysts). Modified Ziehl–Neelsen staining helps detect them. Clinically they cause pulmonary infections (Mycobacterium, Nocardia) or gastrointestinal diarrhoeal disease (Cryptosporidium, Cyclospora).

Detecting any acid‑fast bacillus (AFB) on a sputum smear is considered smear‑positive tuberculosis. Smears are graded by AFB count: scanty = 1–9 AFB/100 fields; 1+ = 10–99 AFB/100 fields; 2+ = 1–10 AFB per field in ≥50 fields; 3+ = >10 AFB per field in ≥20 fields. Molecular tests and culture are used to confirm diagnosis and drug sensitivity.

The acid-fast test detects Mycobacterium tuberculosis in sputum by staining samples (Ziehl–Neelsen or fluorescent methods). Mycobacteria retain dye after an acid-alcohol wash, appearing as acid-fast bacilli under a microscope. It’s quick and inexpensive but less sensitive than culture or molecular tests; positive smears suggest infectious TB and are followed by culture or nucleic acid amplification tests for confirmation and drug-resistance testing.

The GeneXpert test is an automated cartridge-based molecular assay that uses real-time PCR to detect Mycobacterium tuberculosis complex DNA and common mutations conferring rifampicin resistance (rpoB gene). It provides rapid, sensitive results—often within about two hours—from sputum or other specimens, aiding diagnosis of active TB and early detection of drug-resistant disease. Different cartridges exist for other pathogens.

TB sensitivity is determined by drug susceptibility testing (DST). DST uses rapid molecular assays (e.g., Xpert MTB/RIF, line-probe tests) to detect rifampicin and isoniazid resistance, and culture-based phenotypic methods (e.g., MGIT or solid-media proportion tests) to assess susceptibility to first- and second-line drugs. Tests are done on sputum or cultured isolates to guide appropriate treatment.