The Brain-Eating Naegleria fowleri

Jennifer Frazer

Cyst, trophozoite ("amoeba"), and flagellate forms of the protist Naegleria fowleri. Photos by CDC.

Naegleria isn’t a true amoeba.

So why are they called amoebas if they are not? The organisms in question — which, like true amoebas are microbes called protists — do alternate between cysts, flagellate (swimming) forms, and amoeba-like (blobby, crawling) forms that are more properly called “trophozoites”. When times are good, these trophozoites crawl through the mud in search of bacteria to eat. When times are bad, they sprout tails and swim off like guided missiles in search of happier hunting grounds. Either of these forms can rarely, accidentally infect humans, typically in warm, shallow water in the southern U.S. in summertime. When times get *really* bad, they encyst. Click here for a CDC graphic of their life cycle (on the left). But the trophozoite forms only superficially resemble amoebas; their DNA tells us they are something much different indeed.

Naegleria, it turns out, is only a distant relative of the Amoebozoa, the true amoebae, which generally lack flagellae. In fact, the true amoebae seem to be more closely related to fungi and animals than it they are to Heterolobosea, the phylum that includes Naegleria. Naegleria, in turn, seem to be much more closely related to Euglena — the flagellated (tailed) photosynthetic single-celled organisms from high school and college biology lab — and Trypanosoma, the causal organisms of sleeping sickness and Chagas disease. Take a look at this family tree of eukaryotes (nucleated organisms – everything except bacteria and archaea) for the groups Amoebozoa and Heterolobosea to see what I mean.

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