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R. N. Borazjani, S. Kilvington, D. L. Meadows; A Method to Study the Rate of Encystment for Acanthamoeba Spp. and the Effect of Multi Purpose Solutions. Invest. Ophthalmol. Vis. Sci. 2008;49(13):4874.
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Free-living amoebae of the genus Acanthamoeba are common to most natural and manmade aquatic habitats. The organism can cause a blinding keratitis, with contact lens wearers accounting for 90% of reported cases. Acanthamoeba has a life cycle of feeding and dividing trophozoites which, in response to adversity, can form a highly resistant cyst stage. Such cysts can be resistant to Multi Purpose Solutions (MPS) used for contact lens disinfection and the use of one particular MPS has been identified as a significant factor in a recently reported outbreak of acanthamoeba keratitis in the USA. During studies on the physiological response of Acanthamoeba to MPS, it was observed that this particular MPS alone induced trophozoite encystment. Here we report the findings of this study and suggest possible reasons for this phenomenon.
Trophozoites of Acanthamoeba castellanii (ATCC 50370) were inoculated into various commercial (MPS-A to MPS-F) and experimental MPS solutions, and incubated at 28°C for 24 hours. The cells were then treated with detergent to lyse trophozoites but leave cysts intact and viable. The cysts were enumerated by haemocytometer counting and the mean % encystment was calculated relative to the initial trophozoite inoculum. Images from various stages of encystment were recorded with advance light and electron microscopy. Detergent treated cysts were also cultured to determine viability (excystment and trophozoite replication).
After 24 hours, the encystment rate for MPS-F was 28.51+ 8.05 compared to <1% and ≤2% for the other commercial and experimental solutions respectively. Additional studies showed that cyst formation occurred between 6 and 24 hours. The cysts formed were viable as excystment and trophozoite replication occurred on testing.
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