Manual of basic techniques for a health laboratory - libdoc.who.int

5. Bacteriology 223Fig. 5.43 Transferring the specimen to a slideSpecimens from the nose1. Using a small cotton wool swab slightly moistened in sodiumchloride solution, transfer some of the nasal mucus from the plasticsheet to a labelled slide.Fig. 5.44 Mycobacterium lepraeM. leprae rods arranged: (a) in groupsof 2–5 lying in parallel; (b) in largergroups or clusters; and (c) in largenumbers 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 throughthe 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 examinationExamine 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 oftenappear 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 numbersin circular masses called “globi” can be seen (Fig. 5.44 (c)).Note: Nasal smears sometimes contain non-pathogenic acid-fast bacilli that are notM. leprae.Recording the resultsRecord 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 indexThe bacteriological index (BI) is a guide to the bacterial load and is calculatedby adding all the positive findings from all the body sites where a sample hasbeen taken and dividing the total number of positive specimens by the number ofsites. For example: