A simple device for counting microorganisms

Bull. Org. mond. Sant} 1973, 48, 506a507 Bull. Wid Hlth Org.f A simple device for counting microorganisms J. E. BILES 1 Abstract This paper describes a simple but accurate device for counting helminth and schistosome eggs as well as other organisms of similar size that can be suspended in liquids. The device presents the additional ad- vantages of being inexpensive and durable. The need to make accurate counts of fairly large organisms (> 20 ,um) suspended in liquids is met with in many fields. As a result, various devices have been developed to facilitate counting. One of the more commonly used of these devices is the Sedgwick-Rafter counting chamber. This has been found useful for counting eggs of Schistosoma haematobium in urine samples, and Cheever & Powers (1968) have used it to count eggs of S. man- soni in tissue digests and in faecal preparations. Nevertheless the device has not been altogether satisfactory. Accurate filling of the chamber is difficult to accomplish if flooding is to be avoided, and, if it is under-filled, an air space is left under the cover, thus interfering with the count and the volume. Moreover, these counting chambers are expensive and not easily replaced in the field. In the present study, a device made from micros- cope slides was found to offer advantages over the Sedgwick-Rafter counting chamber for counting eggs of S. haematobium. The device is described here in the expectation that it may be found useful by others. The counting device Two portions, each 10 mm long, cut from a stan- dard 25 x 75 mm microscope slide are cemented at each end of an intact slide to form supports (Fig. 1). A single portion of another slide is then cut (the " platform ") and is fastened so as to be equidistant from the supports, thus leaving a space or channel at either end. Another slide is then laid (not fastened) across the supports, creating a space between the upper slide and the platform. The fluid containing the organisms to be counted is introduced into that 1 Technical Officer, Laboratory Operations, Field In- vestigation on Schistosomiasis, Division of Malaria and Other Parasitic Diseases, World Health Organization, 1211 Geneva 27, Switzerland. COVER TfMSUPPORT WHQ J"32 Fig. 1. Construction of the counting chamber. space. Counting is carried out with the aid of a dissecting microscope or a low-power compound microscope. The volume of liquid that can be contained in the space is determined by the length of the platform and the thickness of the glass (all the slides used for making the device should be of uniform thickness). Thus, if the slides have a uniform thickness of 1.5 mm and the platform measures 25 x 45 mm, the counting area will hold about 1.68 cm3. The liquid containing the organisms is delivered into the space by means of a calibrated pipette or any other device capable of delivering a known volume through a narrow opening. Under-filling is no problem because all edges of the " pool " are visible. In the present study, the device was used for counting ova of S. haematobium. The random distribution of these organisms was found to be similar to that observed with the Sedgwick-Rafter chamber, the eggs tending to be more numerous at the point of innoculation. However, as the entire counting area can be examined with the new device, the falling off of ova densities towards the meniscus is accounted for. Epoxy resin cement was found to be useful for assembling the device, but other materials having a refractive index closer to that of glass may be prefer- red in some circumstances. The platform segment may be etched or scratched with a diamond to pro- vide a counting grid, and special slides produced for dark-ground work may be used to give enhanced optical characteristics and a more clear-cut result. The author has found that, by using a diamond object-marker on the microscope, it is very simple to mark an accurate grid on the platform slide. 3040 -506- BRIEF COMMUNICATIONS 507 An even and unchipped line is obtained by means of a spring loaded marker. The mechanical stage is utilised to " track " in a straight line both along and across the slide. After the first line has been drawn, a scanning objective is used to position it at the edge of the field. A second line can then be drawn, and so on. This method produces a very accurate grid and is not time-consuming. A possible further application is in the counting of microfilariae in a blood sample larger than is nor- mally used. Denham et al. (1971) describe another form of chamber, made from slides, for such a purpose. The counting chamber tested in the present study was found to be practical, inexpensive, durable, and accurate. REFERENCES Cheever, A. W. & Powers, K. G. (1968) J. Parasitol., 54, 632-633 Denham, D. A. et al. (1971) Trans. roy. Soc. trop. Med. Hyg., 65, 521-526