Mycobacterium: The Acid-Fast Bacteria and the Diseases They Cause

A laboratory receives a sputum sample and applies a Gram stain, but the bacteria barely take up the dye. Only the Ziehl-Neelsen method, which uses hot carbol fuchsin to drive the stain in, finally colors them red, and then, even when washed with acid-alcohol, they keep the color. That single stubborn property, being acid-fast, identifies the sample containing Mycobacterium, and it comes from a cell wall unlike almost any other bacterium's: thick, waxy, and built from mycolic acid.

That waxy wall is the key to the entire genus. It is why mycobacteria stain as they do, why they grow so slowly, why they survive drying and disinfectants, why they resist many antibiotics, and why the diseases they cause, tuberculosis, leprosy, and others, are chronic and hard to cure. This page is the overview of the genus: what makes mycobacteria unique, how they are classified, and how they are diagnosed.

The waxy cell wall: the key to the whole genus

Mycobacterium is a genus of rod-shaped bacteria defined by one extraordinary feature: a cell wall extremely rich in lipids, above all mycolic acids, long, waxy fatty acids that form a thick, hydrophobic (water-repelling) outer layer. Almost everything about mycobacteria follows from this wall.

• They are acid-fast. The waxy wall resists ordinary stains, including the Gram stain. Special methods (Ziehl-Neelsen, or fluorescent auramine) use heat or strong reagents to drive dye into the cell, and once stained, the cell resists decolorization even by acid-alcohol. This property, holding the stain against acid, is called acid-fastness, and it is the single defining laboratory feature of the genus. (Because they stain so poorly by Gram, mycobacteria are often called "neither Gram-positive nor Gram-negative," though structurally they are closer to Gram-positive.)
• They grow slowly. The waxy wall makes nutrient uptake slow, so mycobacteria divide slowly, M. tuberculosis every 15 to 20 hours (versus 20 minutes for E. coli), which is why cultures take weeks, not days.
• They are environmentally tough. The wall resists drying, acids, alkalis, and many disinfectants, which is why M. tuberculosis survives in dried sputum and airborne droplet nuclei, and why laboratories can use harsh chemicals to decontaminate specimens without killing the mycobacteria.
• They resist many antibiotics. The wall is a barrier that keeps many drugs out, which is part of why mycobacterial infections need multiple drugs for long periods.
• They cause chronic, granulomatous disease. Because they survive inside macrophages and are cleared only slowly, the body walls them off in granulomas, producing the slow, chronic diseases typical of the genus.

If you remember one thing about mycobacteria, remember the waxy mycolic-acid wall. It explains the staining, the slow growth, the toughness, the drug resistance, and the chronic disease, all at once.

General properties of mycobacteria

• Rod-shaped, often slightly curved, sometimes beaded on staining.
• Aerobic and non-motile, non-spore-forming, non-capsulated.
• Acid-fast (the defining feature).
• Slow-growing (most species), requiring enriched media such as Lowenstein-Jensen (LJ) medium and weeks of incubation. A minority (some atypical species) grow faster.
• Catalase-positive (variably), and identified by growth characteristics and biochemical tests, or now by molecular methods.

How the mycobacteria are classified

The genus contains many species, grouped by the disease they cause and their laboratory behavior.

The Mycobacterium tuberculosis complex

A group of closely related species that cause tuberculosis, chiefly:

• Mycobacterium tuberculosis: the main cause of human TB.
• Mycobacterium bovis: cattle TB, transmissible to humans (historically via unpasteurized milk); the source of the BCG vaccine strain.

Tuberculosis is covered on the Mycobacterium tuberculosis and tuberculosis page.

Mycobacterium leprae

Mycobacterium leprae causes leprosy (Hansen's disease). It is unique in that it cannot be grown on any artificial culture medium (it is even more fastidious than the rest of the genus) and grows best at cooler body temperatures, which is why leprosy affects the skin, peripheral nerves, and cooler areas of the body. M. leprae is covered on the leprosy page.

Non-tuberculous (atypical) mycobacteria

The non-tuberculous mycobacteria (NTM), also called atypical or environmental mycobacteria, are a large group found in soil and water that occasionally cause disease, often in people with damaged lungs or weakened immunity. They were classically sorted into four Runyon groups by pigment production and growth rate:

• Group I, photochromogens (pigment only in light), e.g. M. kansasii.
• Group II, scotochromogens (pigment in dark or light), e.g. M. scrofulaceum.
• Group III, non-chromogens (no pigment), e.g. M. avium complex (MAC).
• Group IV, rapid growers (grow in under 7 days), e.g. M. fortuitum, M. abscessus.

These organisms are covered in detail on the atypical mycobacteria page.

The medically important mycobacteria at a glance

How mycobacteria are diagnosed

Because mycobacteria are slow-growing and acid-fast, their diagnosis uses a distinctive set of methods. This article gives the logic; for detailed method, result and interpretation check the given link for each method.

• Acid-fast staining (microscopy). The first and fastest step: staining sputum or tissue to see acid-fast bacilli directly, by the Ziehl-Neelsen technique (hot method) or the more sensitive fluorescent auramine-rhodamine stain. Microscopy is quick and cheap but needs many organisms to be positive, so a negative smear does not exclude TB.
• Culture. The reference standard for confirmation, on solid media such as Lowenstein-Jensen medium or in faster liquid systems. Culture is sensitive and allows drug-susceptibility testing, but is slow (weeks) because the organisms grow slowly.
• Molecular tests. Nucleic acid amplification, above all GeneXpert MTB/RIF, detects M. tuberculosis DNA and rifampicin resistance directly from sputum in hours, transforming rapid TB diagnosis.
• Species identification. Biochemical methods and molecular methods distinguish M. tuberculosis from the atypical mycobacteria.
• Immunological tests. The tuberculin skin test (Mantoux) and interferon-gamma release assays (IGRAs) detect the immune response to TB, indicating infection (latent or active) but not proving active disease.

The full TB diagnostic workflow, how these fit together in practice, is on the M. tuberculosis lab diagnosis page.

Why mycobacterial infections are hard to treat

Two principles apply across the genus, and both come back to the biology above:

• Multiple drugs, for a long time. The waxy wall and slow growth, plus the organism's ability to persist inside cells, mean single drugs fail and short courses relapse. TB and leprosy are treated with combinations of drugs for months, far longer than ordinary bacterial infections.
• Drug resistance is a major problem. Because treatment is long, resistance develops when it is incomplete, giving rise to multidrug-resistant TB (MDR-TB) and worse. This is why supervised, complete, combination therapy is central to TB control.

How to remember

The waxy wall explains everything. Mycolic acid makes the wall thick and waxy, and that one fact explains why mycobacteria are acid-fast (hold stain against acid), slow-growing (nutrients get in slowly), tough (survive drying and disinfectants), drug-resistant (barrier keeps drugs out), and chronic (walled off in granulomas). Learn the wall, and the genus follows.

Acid-fast is the signature. They resist the Gram stain but hold a stain against acid decolorization, that's "acid-fast." If a rod is acid-fast, think Mycobacterium first.

Three buckets. The medically important mycobacteria split into three: the TB complex (M. tuberculosis, M. bovis), M. leprae (leprosy, can't be cultured), and the atypical/environmental ones (opportunists, Runyon groups). Three buckets cover the genus.

Leprae won't grow. M. leprae is the one you cannot culture on any medium, and it likes cool body sites (skin, nerves). If a question says "cannot be grown in the lab," think M. leprae.

Slow to grow, long to treat. Slow growth means cultures take weeks and treatment takes months with multiple drugs. Slow bug, long therapy.

Key exam facts in one table

Where students get confused

"Neither Gram-positive nor Gram-negative." Mycobacteria stain poorly by Gram because of their waxy wall, so they are identified by acid-fast staining instead. Structurally they are closer to Gram-positive, but in practice you never rely on the Gram stain for them.

Acid-fast means holding stain against acid, not being an acid. The term describes the cell keeping its stain when washed with acid-alcohol, because of the waxy wall, not the organism being acidic. This confuses many students constantly.

M. leprae cannot be cultured. Unlike M. tuberculosis (slow but culturable on LJ), M. leprae grows on no artificial medium at all. Expecting a leprosy culture result is a mistake.

Slow growth is a diagnostic problem. Because cultures take weeks, a negative early culture does not exclude TB, and rapid methods (smear, GeneXpert) are needed for timely decisions. Students underestimate how slow the culture is.

Atypical mycobacteria are not TB. The non-tuberculous mycobacteria are environmental organisms causing largely opportunistic disease, and are treated differently from TB. A positive acid-fast smear is not automatically tuberculosis.

References

1. Tille, P. M. (2022). Bailey & Scott's Diagnostic Microbiology (15th ed.). Elsevier.
2. Procop, G. W., et al. (2017). Koneman's Color Atlas and Textbook of Diagnostic Microbiology (7th ed.). Wolters Kluwer.
3. Murray, P. R., Rosenthal, K. S., & Pfaller, M. A. (2020). Medical Microbiology (9th ed.). Elsevier.
4. World Health Organization. (current). Global Tuberculosis Report. WHO.