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

In the press this week were reports (see here and here and here) that the brain-eating amoeba Naegleria fowleri has killed three people this summer, as it does in a typical year. The only trouble is, 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 flagella. 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.

Heterolobosea are difficult to define in a sentence, since they have evolved into so many niches. What tells us they are related is their DNA. But generally speaking, the phylum that includes Naegleria is made up of organisms that alternate between amoeboid and swimming forms, and that, like the Euglenozoa, have "discoid" cristae, or folds, in their energy-producing mitochondria. In contrast, most animals, I believe, have sheet-like or laminar cristae. How exactly discoid is different from laminar I have not been able to discover. For extra randomness, the Heterolobosea also includes a group of slime molds that's completely unrelated to the big showy ones most people (if they ever think of them) think of as slime molds.

The proposed kingdom to which the Heterolobosea belong -- the Excavata -- are even more of a mixed bag, although again one might say they are almost exclusively protists with either no or unusual mitochondria, two or more flagella for swimming, and a characteristic underside feeding groove supported by fibers called microtubules.

There are true amoebas that infect humans. Entamoeba histolytica comes to mind, the cause of the vastly more prevalent disease amoebic dysentary, which infects some 50 million worldwide. Naegleria, on the other hand, are not parasites of humans; they prefer bacteria and don't seek out people. They only infect us when we swim into their habitat and happen to bump into them nose-first. Hungry and far from home, they crawl into our brains and start eating like crazy, killing the unfortunate host 95% of the time.

But as a result of its accidental nature, Naeglaria infection is quite rare in the United States -- happening perhaps 2-3 times a year -- especially compared to organisms that do seek us out in water. As a blog post at the L.A. Times points out:

According to the Centers for Disease Control and Prevention, the most commonly reported recreational water illness (RWI) is diarrhea, which can be caused by germs such as Cryptosporidium, Giardia, Shigella, norovirus and E. coli. These can be introduced into the water through trace amounts of fecal matter that cling to people's bodies. The agency reports:

Swimmers share the water -- and the germs in it -- with every person who enters the pool. On average, people have about 0.14 grams of feces on their bottoms which, when rinsed off, can contaminate recreational water. In addition, when someone is ill with diarrhea, their stool can contain millions of germs. This means that just one person with diarrhea can easily contaminate the water in a large pool or water park.

I think I could have gone my whole life without knowing each and every bottom in the pool contains 0.14 grams of feces. It certainly makes a hard sell for the bidet. This fact alone should be *much* more frightening than the chance that a little, lost, hungry protist might somehow find its way into your brain, no matter how food-crazed it might be once it gets there. And if you're still worried, there's always nose clips.