Our digestive tract harbors trillions of microorganisms that for the most part, happily coexist within us.

This party is going on in the colon, and most of the guests are bacteria. Our microorganisms (a.k.a. “microflora” or “microbiota,”) have been described as being as unique to us as a fingerprint, which makes studying the impact of these tiny life forms on humans an enormous undertaking.

Probiotics are live microorganisms in food or supplements that, when eaten, may promote health. However, there are many hurdles to overcome.

How do intestinal microorganisms help us?

Although not fully understood, they are thought to behave in the following ways.

No Trespassing: Private Property! Microorganisms can set up a blockade-type barrier in the digestive tract to help prevent disease-causing germs from entering the bloodstream. Like animals staking their claim, they compete for nutrients and binding sites, preventing other microorganisms from “sticking” to the surfaces of the digestive tract.

Theoretically, the more “good bugs” we have, the more likely they are to win the war for territory.

Some friendly bacteria may break down potential cancer-causing agents. They also can produce substances which kill harmful microbes and create an acid-like environment that some germs are unable to grow in.

They may reduce inflammation and allergens.

Some products of their metabolism can give us energy for brain, muscle, heart and intestinal tissues.

They can make vitamin K and some B vitamins.

What conditions allow the bad bacteria to outnumber the good ones?

When disease-causing germs find a niche, they can modify our normal balance of “friendly bugs,” giving the bad boys an edge and nudging out the good guys (think food poisoning).

Taking antibiotics can wipe out the good, the bad and the ugly bacteria. Taking probiotics can help to reestablish the good bacteria in your digestive tract. But be sure to take calcium-rich dairy foods a couple of hours before or after taking antibiotics, because the calcium can bind with them.

Are all bacteria created equally?

Not all bacteria are considered to be “probiotic."

These microorganisms are only beneficial to us if they are still around in large quantities when they reach their expiration date. You should select products that are labeled to contain live numbers of microbes “through the end of shelf life.”

Probiotics must be capable of surviving the journey through the acid juices of the stomach and the bile in the small intestines. If they arrive unscathed, they must be able to “go forth and multiply” rather than meeting an untimely death. Many of the starter cultures in yogurt are not considered to be probiotic, because they have expired before they’ve reached the colon.

Because probiotics are regarded as supplements, they do not have to be proven to be effective before they are added to foods. However, disease claims must be backed by scientific data.

Which health conditions do they treat?

Probably the best known use for probiotics is to help people digest the naturally occurring sugar in milk—lactose.

There is also good support for specific strains that treat diarrhea in infants and children, and reduce infants’ risks for developing eczema.

Probiotics also have been reported to improve immune function in older individuals.

Although much of the probiotic research is in the early stages, it is showing some promising results in treating allergies, irritable bowel syndrome, GI inflammation, genitourinary infections, and H. pylori, the microorganism which is often responsible for causing ulcers.

If you think about a shark and your pet guppy, they hardly behave in the same way, even though they are in the same species. The same can be said for various strains of probiotics within the same species, and how they affect different health conditions.

Check with your doctor, pharmacist or dietitian for information about which strains may help with specific health conditions.

Are there any risks involved in taking probiotics?

Two of the most commonly used bacteria are bifidobacterium and lactobacillus, which have a “generally recognized as safe” (GRAS) status in the U.S. They have been safely used for hundreds of years.

People who take probiotics may initially experience gas and bloating, which may or may not subside with continued use.

Parents of premature infants, individuals with weakened immune systems or chronic diseases, and pregnant or nursing mothers should check with their doctor before supplementing their diets with probiotics.

Most of us have heard about germs becoming more resistant to antibiotics, and researchers want to know if this unwelcome characteristic could be passed on from lesser-known probiotics to our own microflora.

Where can I find probiotics?

They are added to many dairy-based foods like yogurt and kefir. You can find them in some types of cereal and granola bars, in juices, and in supplemental forms. Flavorful chewables are making an appearance in stores as well.

Some yogurt manufacturers have incorporated specific strains that will survive as far south as the colon. But these friendly bugs haven’t taken a liking to our accommodations, so they must be ingested for two weeks to be effective. However, a consumer group reports that the scientific evidence for these benefits is weak.

No matter what its bacterial effectiveness, the beauty of yogurt is that it packs a powerful nutritional punch—it’s a great source of calcium, protein, potassium, B vitamins, and sometimes vitamin D, all at a reasonable price. You can’t really lose by eating yogurt—if you try it for two weeks and it doesn’t improve your digestion, you have still benefited by consuming a nutrient-rich food.it

You can find out which supplements pass inspection from Consumer Labs, or check the label for certification from the U.S. Pharmacopeia.

Prebiotics, which are not digested, are much easier to please when it comes to supporting our friendly bacteria. But that’s another story.