Simply Strange: The Story of a Tiny Parasite

N. Parisii Spore Microsporidia — A new species of the tiny parasite was found in roundworms living in Parisian compost. Its spore – the stage that infects a host cell – is show above.

(Image credit: Emily Troemel)

First they were fungi, then protists — and now they are fungi again. Once thought to be primitive, it now seems they have evolved backward, becoming simpler rather than more complex.

Microsporidia — single-celled parasites that include bugs implicated in the disappearance of honey bees — are strange. So far, about 1,300 species have been formally described, according to Patrick Keeling, a professor at the University of British Columbia who studies them. They are known to infect fish, birds, insects and even us, and Keeling only expects to see their ranks grow.

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The first microsporidian identified was found infecting silkworms. In 1870, Louis Pasteur, the French chemist who invented the process of pasteurization, blamed it for a disease that had decimated the industry. Initially, they were classified as fungi, then renamed protists, a catch-all group for complex micro-organisms. As protists, they spent the better part of the 20th century in what Keeling refers to as a "grab bag," an unnatural group containing single-celled parasites later reclassified as animals and green algae. [10 Most Diabolical and Disgusting Parasites]

But new molecular evidence eventually challenged this, culminating in the discovery of tiny relics of mitochondria, called mitosomes, within microsporidia, according to Keeling. Rather than being primitive, it seems these organisms have become simpler — one could say more streamlined — than their ancestors. As a result of the new information, the tiny parasites were also, eventually, reinstated as fungi, which are complex organisms that cannot generate their own food, unlike plants. The group includes yeast, mold and toadstools.

Microsporidia only grow inside a host organism's cells, making them much trickier to study than other pathogens, like most bacteria, which can be grown in a petri dish and manipulated. Troemel's lab is getting around this problem by looking at transparent roundworms whose guts have been infected by a microsporidian dubbed Nematocida parisii, or "nematode killer from Paris," because it was discovered in roundworms found in Parisian compost pits.

Microsporidia have an appropriately dramatic technique for infecting a cell. Their spores, the only stage to survive outside a host cell, have a thick, rigid wall with a tube coiled up inside it. In response to cues that are poorly understood, the spore ruptures its wall and the tube shoots out like a harpoon, and pokes into the future host cell. The remaining contents of the spore, including its command center, the nucleus, are then pumped through the tube into the host cell.

Jerry Bromenshenk, a research professor at the University of Montana, and colleagues have linked vanishing colonies in North America with a virus-fungal tag team. The virus is insect iridescent virus, named for the effect it creates in infected tissues, and the fungi is Nosema ceranae, a microsporidian. In a study published in October 2010 in the journal PLoS ONE, the team described finding this combo in bees from failing colonies, and by testing both pathogens in bees, they found that together, they were more lethal than if they infected bees separately.

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Wynne was a reporter at The Stamford Advocate. She has interned at Discover magazine and has freelanced for The New York Times and Scientific American's web site. She has a masters in journalism from Columbia University and a bachelor's degree in biology from the University of Utah.