A 30-year old patient presented with features of acute meningoencephalitis in the casualty. His CSF on wet mount microscopy revealed motile unicellular microorganisms. The most likely organism is -

Provided history of acute meningoencephalitis with demonstration of motile trophozoites in CSF wet mount suggests diagnosis of N.fowleri. Eventhough Acanthamoeba also causes meningoencephalitis, it is not acute in nature and it occurs mainly in immunocompromised host.