Mycobacterium leprae Bacteriology
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Direct answer
Mycobacterium leprae is an acid-fast, obligate intracellular bacillus that cannot be grown in routine cell-free culture media, so leprosy diagnosis rests on clinical signs and slit-skin smears. The organism multiplies best at about 30 degrees Celsius and divides very slowly, with a doubling time of around two weeks. Experimental cultivation uses the foot pads of mice and the nine-banded armadillo.
What you must remember
- Morphology: an acid-fast rod that often forms cigar-shaped bundles called globi inside macrophages.
- Obligate intracellular: it lives inside macrophages and Schwann cells, which explains its affinity for skin and peripheral nerves.
- Growth temperature: multiplication is best at about 30 degrees Celsius, favouring cooler sites such as skin, nerves and the earlobes.
- Doubling time: roughly two weeks, one of the slowest of any bacterium, which explains the long incubation of leprosy.
- Bacterial index: a grade of bacillary load in slit-skin smears on a logarithmic scale from 0 to 6 plus.
- Morphological index: the percentage of solid-staining, viable bacilli among all bacilli counted.
- Lepromin test: an intradermal test read after several weeks, positive in tuberculoid leprosy and negative in lepromatous leprosy.
Reading a slit-skin smear step by step
A patient has a hypopigmented skin patch with reduced sensation, and the clinician takes slit-skin smears to grade the bacillary load. The steps show how the bacterial index and the morphological index are generated.
- Choose the sites: smears are taken from the earlobes and from one or two active lesions, after a small slit releases tissue fluid from the dermis.
- Stain the smear: the smear is fixed and stained by Ziehl-Neelsen, and the bacilli appear as red acid-fast rods, often clumped together.
- Count the bacilli: a grid of fields is examined, and the average number of bacilli per field is recorded on a logarithmic scale from 0 to 6 plus.
- Calculate the morphological index: the proportion of solid-staining, viable bacilli among all bacilli counted is recorded as a percentage.
- Interpret the indices: a high bacterial index and morphological index indicate lepromatous leprosy with a heavy bacillary load, whereas a zero bacterial index suggests tuberculoid disease.
- Track treatment response: a falling morphological index during treatment shows that bacilli are being killed, while the bacterial index falls slowly in lepromatous disease.
Where the exam tries to catch you
Leprosy bacteriology questions test the difference between diagnosis, classification and experimental culture.
- Trap: using the lepromin test to diagnose leprosy Correct: it measures cell-mediated response and helps classify disease and predict prognosis, but diagnosis is clinical.
- Trap: assuming Mycobacterium leprae grows on Lowenstein-Jensen medium Correct: it cannot be grown on routine media, so the armadillo and mouse foot pad are used for experimental work.
- Trap: confusing the bacterial index with the morphological index Correct: the bacterial index measures total bacillary load on a logarithmic scale, whereas the morphological index measures the proportion of viable bacilli.
Frequently asked questions
Why cannot Mycobacterium leprae be cultured in routine laboratory media?
Mycobacterium leprae is an obligate intracellular organism that has lost many genes through reductive evolution. It therefore cannot grow in routine cell-free culture media, which is why it is grown experimentally in armadillos and mouse foot pads.
What is the bacterial index in leprosy?
The bacterial index is a semi-quantitative grade of bacilli in slit-skin smears, recorded on a logarithmic scale from 0 to 6 plus. Higher values indicate a heavier bacillary load, which is typical of lepromatous leprosy.
What is the morphological index in leprosy?
The morphological index is the percentage of acid-fast bacilli that are solid-staining, and therefore viable, among all bacilli counted. A falling morphological index during treatment indicates that the bacilli are being killed.
What does the lepromin test show in leprosy?
The lepromin test assesses cell-mediated immunity to Mycobacterium leprae and is positive in tuberculoid leprosy but negative in lepromatous leprosy. It helps classify disease and predict prognosis, but it cannot diagnose leprosy on its own.
Which drugs are used for paucibacillary leprosy under multidrug therapy?
Under India's NLEP, paucibacillary leprosy now receives rifampicin, dapsone and clofazimine for six months, while multibacillary leprosy receives the same three drugs for 12 months (uniform MDT from April 2025). Older textbooks list two drugs for paucibacillary disease, so the bacillary classification still decides the duration.