Laboratory features of M.leprae
Morphology
M. laprae is a non-motile, non-sporing, straight or slightly curved rod measuring 0.2-0.5 x 5-8μm bacteria. Pointed or enlarged ends are sometimes seen. The organisms can be found singly, in clusters, and in large groups within macrophage cells.
Method of collecting and examining a slit skin smear for M.leprae
A trained and experienced observer using an aseptic and safe technique must collect a smear for the examination of M.leprae.
The skin should be squeezed well and small incision made with sharp blade and the tissue fluid is scraped by turning the blade at a right angle. The specimen is smeared on glass slide.
Sites for slit skin smear include eye brow, ear lobe, elbow gluteal area, knees and active edge of a lesion.
Procedure
1. Explain the procedure to the patient (or parent if the patient is a child) The patient should sit with his or her back to the table on which the equipment for taking the smear is placed.
2. Fit a new scalpel blade in its scalpel holder.
Sterilize the blade by wiping it carefully with a piece of absorbent cotton wool soaked in 70% ethanol (alcohol) and flaming it for 2-3 seconds in the flame of a spirit lamp.Allow the blade to cool; making sure it is not touching any unsterile surface.
3. Wearing protective rubber gloves, cleanse the area from where the smear is to be taken (e.g. earlobe), using a cotton wool swab moistened with 70% ethanol (alcohol). Allow the area to dry.
4. Pinch the skin tightly between the thumb and index finger until it becomes pale due to loss of blood.
- Important: The area must be kept bloodless while the smear is collected because a smear, which contains red cells, will be difficult to examine and report.
5. Using the sterile blade, make a small cut through the skin surface, about 5mm long and deep enough in to the dermis (2-3mm) where the bacteria will be found. Continue to hold the skin tightly.
6. Using a dry piece of cotton wool, blot away any blood, which appears at the site of the cut.
Note: providing the pressure is maintained between the thumb and index finger, little or no bleeding should occur.
7. Turn the scalpel blade until it is at a right angle to the cut.
Using the blunt edge of the blades, scrape firmly two or three times along the edges and bottom of the cut to collect a sample of tissue juice and cells
8. Transfer the sample to a slide. Make a small circular smear, covering evenly an area measuring 5-7 mm in diameter.
9. Cover the cut with a small dressing. Instruct the patient to remove the dressing as soon as the cut has healed
10. Ensure the slide is clearly labeled with the patient’s name and identification number
11. When the smear has dried, gently heat-fix by holding the smear upper most over the flame of a spirit lamp or the pilot flame of a Bunsen burner for a few seconds.
Do not over – heat because this will interfere with the staining of M.leprae.
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