Rapid detection of Acanthamoeba cysts in corneal scrapings by chlorazol black E staining

Several staining techniques to enhance the visibility of Acanthamoeba cysts

Acanthamoeba is one of the most common free-living amoebae. It is widespread in the environment and can infect humans causing keratitis. Delayed diagnosis or misdiagnosis leads to extensive corneal inflammation and profound visual loss. Therefore, accurate and rapid diagnosis of Acanthamoeba keratitis is essential for successful treatment and good prognosis. This study was designed to use different staining techniques to facilitate the identification of Acanthamoeba cysts. Acanthamoeba cysts were isolated by cultivation of either corneal scraping specimens or tap water samples onto non-nutrient agar plates seeded with Escherichia coli. Subcultures were done from positive cultures until unique cysts were isolated. Acanthamoeba cysts were stained temporarily using iodine, eosin, methylene blue, and calcofluor white (CFW) stains and as permanent slides after processing for mounting using modified trichrome, Gimenez and Giemsa staining. These stains were compared on the basis of staining quality including clarity of morphological details, differentiation between cytoplasm and nuclei, color and contrast, and also other characteristics of the staining techniques, including ease of handling, time taken for the procedure, and cost effectiveness. The cysts of Acanthamoeba were recognized in the form of double-walled cysts: the outer wall (ectocyst) that was being differentiated from the variably stained surrounding background and the inner wall (endocyst) that was sometimes stellated, polygonal, round, or oval and visualized as separate from the spherical, sometimes irregular, outline of the ectocyst. Regarding the temporary stains, it was found that they were efficient for visualizing the morphological details of Acanthamoeba cysts. In CFW staining, Acanthamoeba cysts appeared as bluish-white or turquoise oval halos although the internal detail was not evident. On the other hand, the results of permanent-stained slides showed the most consistent stain for identification of Acanthamoeba cysts was modified trichrome followed by Gimenez stain and lastly Giemsa stain that gave poor visibility of Acanthamoeba cysts due to the intense staining background and monochrome staining of parasite. In the present study, multi-attribute ranking of the used staining techniques showed the highest rank for iodine stain (92 %) followed by eosin stain (84 %), Gimenez stain (76 %), methylene blue (72 %), CFW (64 %), modified trichrome (56 %), and the least was Giemsa stain (44 %). In conclusion, the staining techniques enhance the overall visibility of Acanthamoeba cysts.