Farmer ants have bacterial farmhands.

A graduate student who once mused about funny-looking patches on ants has discovered that the insects have a microscopic partner species overlooked despite about a century of study.

Researchers have long known that the tribe of New World ants literally farms gardens of spongy fungus to feed themselves. Now it turns out that the ants get help from a mutualistic relationship with Streptomyces bacteria, report Cameron R. Currie of the University of Toronto and his colleagues.

Whitish patches of bacteria on the ants secrete antibiotics that target a major pest in their gardens, the researchers say in the April 22 NATURE. This weed fungus, Escovopsis, can reduce an ant garden to brown ooze in just a few days.

More than 2,000 research papers have focused on the weed biology of ants and the food fungi they cultivate, Currie says (SN: 11/21/98, p. 334). Yet previous studies completely missed the bacteria and noted the weed fungus only as a rarity, not the widespread menace that it has proved to be.

The new work "totally changes this textbook relationship from two to four players," Currie points out. He says he wouldn't be surprised if other classic mutualisms turn out to include overlooked microbes.

Other scientists had noticed the bits of pale crust on the ants but dismissed them as an ant secretion. When Currie took a fresh look, he realized the patches were alive. The research team identified them as Streptomyces, the same genus that has yielded many antibiotic drugs. Extracts from the patches did nothing to most fungi tested but slammed Escovopsis.

The Streptomyces bacteria turned up on all 22 species of fungus growers that Currie checked. This survey covered 8 of the fungal-farming 12 genera, from just-the-basics farmers to leaf cutters that snip up foliage to feed their fungi.

All 74 young ant queens that Currie examined carried bacterial helpers as they set off to start their own fungus farms in new nests. Males, which don't farm, did not sport bacteria.

The Escovopsis fungus seems to prey only on ant gardens, Currie says, and he found it in many of the colonies he examined. "It's an ancient coevolutionary arms race," he says. Countering the weed is an alliance of ants, their food fungus, and bacterial helpers.

Ted Schultz of the Smithsonian Institution in Washington, D.C., welcomes the new work as "a spectacular paper" raising many new questions about the bacteria and the weed. "It opens up a whole lot of neat ecology," Schultz says.

Ulrich G. Mueller of the University of Maryland in College Park comments, "What's interesting from an evolutionary perspective is that once again the ants hit on something before we did." Ants beat humans in developing agriculture by some 50 million years. Now, he says, it looks as if the same ants came in ahead on bacterial antibiotics by millions of years.