Mycobacteria
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Direct answer
Acid-fastness — carbol fuchsin driven into the waxy, mycolic-acid-rich wall by heat and then refusing to wash out with acid–alcohol — is the property that defines mycobacteria on a smear, counterstained blue-green with methylene blue or revealed as yellow rods on auramine–rhodamine fluorescence. Mycobacterium tuberculosis grows on Lowenstein–Jensen medium in 2–8 weeks, which is why India's National Tuberculosis Elimination Programme now makes the molecular test (CBNAAT, the Cartridge-Based Nucleic Acid Amplification Test) the first diagnostic for every presumptive case, returning rifampicin resistance within two hours. The nontuberculous mycobacteria sort by Runyon's classification — photochromogens, scotochromogens, non-chromogens and rapid growers. Mycobacterium leprae remains uncultivable in artificial media, diagnosed by slit-skin smear and skin biopsy with lepromin testing used only to classify, never to diagnose.
What you must remember
- Ziehl–Neelsen: primary stain carbol fuchsin with heat, decolourise with 20 per cent sulphuric or acid–alcohol, counterstain methylene blue; the Kinyoun method does it cold with higher phenol.
- CBNAAT on sputum (or extrapulmonary specimens) detects M. tuberculosis and rifampicin resistance within about two hours and is the NTEP's first test; liquid culture (MGIT) remains the reference standard and pans the susceptibility picture.
- Bacille Calmette–Guérin is a live attenuated Mycobacterium bovis strain, given at birth intradermally in the left deltoid in India; it protects against miliary and meningeal disease far better than against adult pulmonary disease.
- Runyon groups: I photochromogens (M. kansasii, M. marinum), II scotochromogens (M. scrofulaceum), III non-chromogens (M. avium–intracellulare, the classic disseminated infection below a CD4 count of 50), IV rapid growers (M. fortuitum, M. chelonae, M. abscessus — visible within a week).
- M. leprae: obligate intracellular, uncultivable, grows in mouse footpads and the nine-banded armadillo; maximum generation time of any common bacterium, about 12–14 days.
- Lepromin test measures host cell-mediated immunity, not infection: strongly positive in tuberculoid leprosy, negative in lepromatous — and it plays no role in diagnosis.
- Multibacillary cases (more than five skin lesions or smear-positive) receive the three-drug rifampicin–dapsone–clofazimine blister; paucibacillary cases receive rifampicin plus dapsone, dispensed free through the National Leprosy Eradication Programme.
- Typical Indian exam nuance: “fluffy, corded, buff-coloured colonies at 4–6 weeks on LJ” is the culture phrase; niacin positive, nitrate positive and catalase negative at 68°C identify M. tuberculosis sensu stricto.
From sputum to programme decision
A 45-year-old with a month of evening fever and cough produces two sputum samples. In the programme pathway both go for CBNAAT rather than the older smear-first algorithm: a positive result with rifampicin susceptibility detected starts the standard drug-susceptible regimen and Nikshay notification the same day, while a rifampicin-resistant result routes the patient urgently to drug-resistant tuberculosis services for a full lineage of testing before a regimen is built. Smear microscopy still matters — two grading signs, and it tracks treatment response at the end of the intensive phase — and culture remains in play for extrapulmonary or smear-negative doubt, with liquid systems flagging growth in about two weeks instead of six. Histology from a lymph node adds caseating granulomas with Langhans giant cells when tissue is the question. The discipline of the pathway — molecular first, culture as reference, smear for follow-up — is precisely what the exam describes when it asks for the “initial investigation of choice under NTEP”.
Where students slip
Two diagnoses are wrongly treated as interchangeable with tests: lepromin is a classification and prognostic tool, so a stem using it to “confirm leprosy” is a distractor — slit-skin smear and histology diagnose. Similarly, the anaesthetic hypopigmented patch of tuberculoid leprosy is usually smear negative, so a negative smear never excludes the diagnosis. The second recurring slip is Runyon rote without the logic: pigment needs light in group I, appears in the dark in group II, never appears in group III, and group IV is defined by speed, not colour. Third, examinees quote culture as the fastest answer — it is the slowest of the three, and the two-hour CBNAAT exists precisely to correct that delay.
Frequently asked questions
Why are mycobacteria acid-fast?
Their cell wall is rich in mycolic acids bound to arabinogalactan, making the wall hydrophobic; once carbol fuchsin penetrates with heat, acid–alcohol cannot strip it out.
What is the first diagnostic test for tuberculosis under NTEP?
CBNAAT on the specimen, detecting M. tuberculosis and rifampicin resistance in about two hours; liquid culture remains the reference standard and smears track treatment response.
Which mycobacteria are rapid growers?
Runyon group IV — M. fortuitum, M. chelonae and M. abscessus — producing visible colonies within about a week and causing soft-tissue, wound and catheter infections.
What does the lepromin test actually measure?
Host cell-mediated immunity to M. leprae: strongly positive in tuberculoid disease, negative in lepromatous — a classification and prognostic marker, never a diagnostic test.
Why is Mycobacterium leprae difficult to culture?
It is an obligate intracellular parasite with a generation time of about two weeks, cultivable only in mouse footpads and the nine-banded armadillo, never on artificial media.