Bacterium From Hell

MOST BACTERIA HAVE THE DECENCY TO BE MICROscopic. Epulopiscium fishelsoni is not among them. The newly identified one-celled macro-microorganism, which lives harmlessly in the intestine of the Red Sea-dwelling brown surgeonfish, is a full fiftieth of an inch long, large enough to be seen with the naked eye. Described in the current Nature, it is a million times as massive as the bacteria that inhabit the human gut.

Epulopiscium is notable for sheer grotesqueness, of course, but it also upsets some long-held scientific assumptions. For one, biologists had believed that bacteria could never be very large because, unlike one-celled animals (such...