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5. Bacteriology 223
Fig. 5.43 Transferring the specimen to a slide
Specimens from the nose
1. Using a small cotton wool swab slightly moistened in sodium
chloride solution, transfer some of the nasal mucus from the plastic
sheet to a labelled slide.
Fig. 5.44 Mycobacterium leprae
M. leprae rods arranged: (a) in groups
of 2–5 lying in parallel; (b) in larger
groups or clusters; and (c) in large
numbers in circular masses (globi).
2. Spread the material evenly on the slide and leave to dry.
3. When completely dry, fix the slide by passing the back of the slide quickly through
the flame of a spirit lamp or Bunsen burner several times.
4. Stain the slide by the modified Ziehl–Neelsen technique (see section 5.3.3).
5.12.3 Microscopic examination
Examine the slide using the ¥ 100 oil-immersion objective.
Mycobacterium leprae are acid-fast bacilli. After staining by the modified Ziehl–
Neelsen technique, they appear red on a blue background.
Size: 1–8mm.
Shape: Largish rods, straight or slightly curved with rounded ends; they may often
appear granular with the rod being broken into several parts.
Arrangement: The rods are arranged either in groups of 2–5 lying in parallel (Fig.
5.44 (a) or in larger groups or clusters (Fig. 5.44 (b)); occasionally large numbers
in circular masses called “globi” can be seen (Fig. 5.44 (c)).
Note: Nasal smears sometimes contain non-pathogenic acid-fast bacilli that are not
M. leprae.
Recording the results
Record the results as follows:
— acid-fast bacilli present, or
— no acid-fast bacilli seen.
The results of the examination can be graded as shown in Table 5.3.
Bacteriological index
The bacteriological index (BI) is a guide to the bacterial load and is calculated
by adding all the positive findings from all the body sites where a sample has
been taken and dividing the total number of positive specimens by the number of
sites. For example: