Bacteria aren't usually welcome ingredients in food or drink. Just ask anyone who has recently had to toss out a jar of peanut butter because of concerns that it might be tainted with salmonella.

But some strains of friendly bacteria and other microorganisms are shaping up to be potentially smart additions to healthy diets.

In recent years, these bacteria have captured both scientific and public attention. The buzz about them "has become a roar," concludes a 2006 report issued by the American Academy of Microbiology, noting that hundreds of foods and dietary supplements are touting on their labels the bacteria they contain.

The products are so popular that they rank among "the top five foods that people say they want to add to their diets," says Harry Balzer, vice president of the NPD Group, a Chicago-based company that tracks consumer trends. (The others, in order of preference, are whole grains, dietary fiber, antioxidants and omega-3 fatty acids.)

Yet despite this growing popularity, and the willingness to pay extra for food and drink that contain the friendly microbes, Balzer notes that "most people don't know what they are."

And the specific strains that show such promise in the laboratory and in clinical studies are rarely, if ever, the same friendly bacteria that are appearing in your food, drink or dietary supplements.

Scientists call them probiotics, prebiotics and synbiotics. Probiotics contain "friendly" bacteria, which you may already be getting if you eat yogurt, drink kefir or sip buttermilk. Probiotics go further: They contain strains of bacteria that convey health benefits when eaten regularly and in high enough quantities. These days you can find them in foods -- Stonyfield and Activia yogurts -- as well as in the dairy drink, DanActiv and several dietary supplements.

Prebiotics are plant substances, such as the inulin found in chicory, onions, garlic, artichokes, bananas, wheat and asparagus. Poorly digested by the stomach, these prebiotics nourish friendly bacteria in the intestine, allowing them to multiply and squeeze out other harmful microorganisms.

And those synbiotics? They're a combination of both friendly bacteria and the food that enables them to thrive. They're not usually found naturally in foods, but are now added to products. One example: the yogurt Activia Light.

Where these microbes seem to have the most benefit is in treating or preventing a host of gastrointestinal problems, from the diarrhea that often occurs with antibiotic use to alleviating irritable bowel syndrome. There's also evidence that they may help prevent recurrence of bladder cancer as well as urinary tract and vaginal infections.

Some friendly bacteria treat pouchitis, a painful inflammation that can occur after the colon is removed because of ulcerative colitis. Other strains appear to help prevent and treat the itchy skin condition known as eczema in both infants and adults.

And a report published this month by Irish researchers suggests that synbiotics may help cut the risk of additional tumors in people with colon cancer.

"There's enough science to suggest that adding a little bit of these foods can help," says Mary Ellen Sanders, co-founder and past president of the International Scientific Association for Probiotics and Prebiotics.

But whether these friendly microbes also are being oversold is a concern of scientists, who say there's need for much more regulation of the products now flooding shelves and dairy cases. The quality of the products available to consumers "is unreliable," the Academy of Microbiology concluded in its report.

As scientists point out, just because a bacteria is "friendly" in the laboratory doesn't mean that it can act as a probiotic in the body. "It's like saying you know Brad Pitt," explains Gregor Reid, professor of microbiology, immunology and surgery at the University of Western Ontario. "Well, if the Brad Pitt you know is not the Hollywood actor, but some guy who lives in Atlanta, it's meaningless, because I'm expecting that you're talking about the Hollywood actor.''

While food labels sometimes show the type of bacteria a product contains, there's no way for you to know the exact strain or how much you're getting. Food companies consider that information proprietary. "That's like saying that the amount of calcium in a product is a trade secret," Sanders says.

So what should you do in the meantime? Stick with a variety of food sources that contain these friendly microbes and buy them only from large, well-known companies. "That," Sanders says, "is the best advice for now."

Join Sally Squires online from 10 to 11 a.m. Tuesdays at www.leanplateclub.com, where you also can subscribe to the free Lean Plate Club weekly e-mail newsletter.