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

216 Manual of basic techniques for a health laboratory5.9 Examination of watery stool specimensDark-field microscopy is used to identify Vibrio cholerae and Campylobacter spp. inwatery stool specimens.5.9.1 Materials and reagents●●●●Microscope with dark-field attachmentMicroscope slidesCoverslipsInoculating loop● Sodium chloride, 0.85% solution (reagent no. 53).5.9.2 Method1. Suspend 0.2g of stool in 5ml of sodium chloride solution. Allow the large particlesto sediment.2. Using an inoculating loop (sterilized by flaming), prepare a very thin smear on aslide. Carefully remove any large particles.3. Cover with a coverslip. Place the slide on the microscope stage.4. Open the iris diaphragm fully and place the dark-field attachment in position.5.9.3 Microscopic examinationUse the ¥ 10 objective for focusing. The background appears black, and all objectssuspended in the saline solution appear bright.Use the ¥ 40 objective to search for bacteria with characteristic shapes and motility(see below).Vibrio cholerae appears as motile rods, which may be short, curved, straight or involuted(Fig. 5.34).Campylobacter spp. are Gram-negative spiral rods that rotate rapidly on a centralaxis.5.9.4 Dispatch of specimens for culture 1It is often necessary to send stool specimens to a bacteriology laboratory forculture:— for the detection of cholera vibrios— for the detection of other bacteria that cause dysentery (species of Salmonella,Shigella, etc.).Using Cary–Blair transport mediumCary–Blair transport medium (reagent no. 17) will preserve many kinds of entericbacteria (cholera vibrios, other vibrios, salmonella, shigella, etc.) for up to 4 weeks.The uninoculated medium may be stored in a sealed bottle at room temperaturefor 8–12 weeks.1. Dip a sterile cotton wool swab in the stool specimen (Fig. 5.35).2. For infants or other patients who cannot produce a stool specimen, take a rectalswab. Moisten the swab with sodium chloride solution and introduce the swabinto the rectum. Turn the swab several times with a circular movement (Fig.5.36).1See also section 3.7.1.