Have you ever opened a bottle of wine and thought you’d stepped in something? That pungent aroma, politely called “barnyard,” is a telltale sign of Brettanomyces, a microbial spoiler of wine. Brett, as it is commonly called in wine circles, is also responsible for wet dog, sweaty horse and band-aid, among other unappetizing aromas and flavors.

“When you give people a bretty wine to smell, they associate it with France, and of course the French don’t like that,” says Linda Bisson, a professor of viticulture at the University of California at Davis. “They associate it with spoilage.”

Brett is a yeast that can grow in winery hoses, barrels, air vents — almost anywhere, even in a wine after bottling. It thrives on the phenolic compounds essential to red wines and does its dirty work after the beneficial yeast Saccharomyces has finished fermenting the grapes’ sugar into alcohol.

Brett has been vilified in modern winemaking since the 1950s, when it was first identified as a spoilage agent. UC-Davis, with its emphasis on winery hygiene and sanitation, has fought against various spoilers, including brett. Brett can be controlled by judicious use of sulfur dioxide during winemaking or eliminated by filtration before bottling. Many wineries guard against it by adding dimethyl dicarbonate, brand name Velcorin, a controversial chemical that can be toxic when undiluted but breaks down harmlessly in solution once added to wine.

Now Bisson and her UC-Davis colleagues are questioning the conventional wisdom that brett is bad. Or at least that all brett is bad. They tested more than 80 strains and discovered that some are specific to different regions and can vary from winery to winery. And about a quarter of those strains add good flavors — including some that wine lovers (and writers) extol.

“Brett gives floral and fruity flavors as well, and we wanted to discover exactly what those are,” Bisson told me. She and fellow UC-Davis viticulturist Lucy Joseph developed a Brettanomyces “flavor wheel,” similar to the famous UC-Davis aroma wheel that has helped educate U.S. wine lovers for three decades. Along with the familiar brett flavors of rancid meat, sewer gas, urine, vomit and burnt beans are the more soothing violets, lavender, chocolate, tobacco and coffee. Many of the flavors typically credited to oak barrels and even the grapes themselves are attributed here to brett.

Their conclusions challenge the basic paradigm of modern wine. When Bisson and Joseph presented their findings at Davis this year, wine writer W. Blake Gray attended and participated in a Brettanomyces tasting. He wrote in the online publication Palate Press, “It’s like learning that Darth Vader is my father.”

So that wonderful smoky, leathery character of Loire Valley cabernet franc that I call “Grandpop’s library” isn’t really part of the mystical terroir of that romantic wine region?

“Nope. That’s brett!” Bisson chirped, happily relegating my fanciful notion to a petri dish.

But more than challenging my preconceptions, Bisson also takes issue with popular winemaking conventions. If these flavors are coming from yeast present in the winery, then what exactly is terroir?

“If you’re getting characters that come from the yeast, you’re not showcasing the vineyard; you’re showcasing conditions in the winery,” she says. “It’s not just geography and climate, it’s winemaking decisions. It flies in the face of the idea that wine is made in the vineyard, if you have the impact of organisms in the winery.”

Chris Howell, winemaker at Cain Vineyards in Napa Valley, welcomes a modicum of brett in his wines. He says the yeast gives his cabernet sauvignon a savory mouth feel and “animalistic” flavors that add complexity. And he points out that brett is an important contributor to the flavor of Belgian lambic beer.

“We have to get beyond the idea of total control,” Howell said during a recent visit to Washington. “Otherwise, we’re just a brand.”

Bisson says she plans to continue researching brett’s effect on wine’s flavors. She quoted a theory that some strains contribute good flavors early in the winemaking process, so winemakers may not recognize brett’s influence in time to take action before the flavor turns fecal. And she noted that even at low levels, brett seems to have trouble integrating successfully with overtly fruity wines.

“When it’s well integrated, brett’s fine,” she says. “But when it’s on top of fresh strawberries, you wonder, ‘Who stubbed out their cigarette in my bowl of strawberries?’ ”

McIntyre blogs at dmwineline.com. On Twitter: @dmwine.