Microbiology of M.leprae
The aetiological agent of leprosy is Mycobacterium leprae. It is a strongly acid-fast rod-shaped organism with parallel sides and rounded ends. In size and shape it closely resembles the tubercle bacillus. It occurs in large numbers in the lesions of lepromatous leprosy, chiefly in masses within the lepra cells, often grouped together like bundles of cigars or arranged in a palisade. Chains are never seen. Most striking are the intracellular and extra-cellular masses, known as globi, which consist of clumps of bacilli in capsular material. Under the electron microscope the bacillus appears to have a great variety of forms. The commonest is a slightly curved filament 3-10 m in length containing irregular arrangements of dense material sometimes in the shape of rods. Short rod-shaped structures can also be seen (identical with the rod-shaped inclusions within the filaments) and also dense spherical forms. Some of the groups of bacilli can be seen to have a limiting membrane.
It is believed that only leprosy bacilli which stain with carbol-fuchsin as solid acid-fast rods are viable and that bacilli which stain irregularly are probably dead and degenerating. The differences are valuable pointers in biopsy specimens to the effects of treatment. In patients receiving standard multidrug therapy (MDT), a very high proportion of bacilli are killed within days, which suggests that many of the manifestations of leprosy, including reactions of the erythema nodosum type, which follow initial treatment, must be due in part to antigens from dead organisms rather than living bacilli. We therefore need drugs which will help the body to dispose of dead but still intact leprosy bacilli.
Two indices which depend on observation of M. leprae in smears from skin or nasal smears are useful in assessing the amount of infection, and the viability of the organisms and also the progress of the patient under treatment. They are the morphological index and the bacteriological index.
The bacteriological index (BI)
This is an expression of the extent of bacterial loads. It is calculated by counting six to eight stained smears under the 100 x oil immersion lens. in a smear made by nicking the skin with a sharp scalpel and scraping it; the fluid and tissue obtained are spread fairly thickly on a slide and stained by the Ziehl-Neelsen method and decolorized (but not completely) which 1% acid alcohol. The results are expressed on a logarithmic scale.
- 1+ At least 1 bacillus in every 100 fields.
- 2+ At least 1 bacillus in every 10 fields.
- 3+ At least 1 bacillus in every field.
- 4+ At least 10 bacilli in every field.
- 5+ At least 100 bacilli in every field.
- 6+ At least 1000 bacilli in every field.
The bacteriological index is valuable because it is simple and is representative of many lesions but is affected by the depth of the skin incision, the thoroughness of the scrape and the thickness of the film.
A more accurate and reliable index of the bacillary content of a lesion is given by the logarithmic index of biopsies (LIB. These indices help to assess the state of patients at the beginning of treatment and to assess progress.
The morphological index (MI)
This is calculated by counting the numbers of solid-staining acid-fast rods. Only the solid-staining bacilli are viable. It is not unusual for solid-staining M. leprae to reappear for short periods in patients being successfully treated with drugs. It is important to recognize that measurement of MI is liable for observer variations and therefore not always reliable.