Probing Question: What are probiotics?

yogurtStephanie Obodda

Among so-called functional foods—fresh or processed items that promise health benefits beyond basic nutrition—few are as popular or as venerable as cultured dairy products that contain probiotics. But just what are these highly touted microorganisms?

"Probiotics are living bacteria that when ingested with food or a nutritional supplement take residence in our gut, mainly in the lower intestines and may influence health in general and specifically," says Penn State food scientist Bob Roberts.

Fermented foods have been consumed by humans since ancient times in the form of yogurt and kefir, and non-dairy foods such as miso, tempeh, and sauerkraut. The Russian physiologist and Nobel-prize winner Elie Metchnikoff (1845-1916), was the first to suggest that consuming bacteria could have a favorable effect on health. Metchnikoff had observed the longevity and robust health of the yogurt-eating Balkan peoples of Eastern Europe.

It is only recently, however, that researchers have begun to demonstrate specific uses of probiotic foods. After birth, Roberts explains, our sterile guts start acquiring bacteria from milk and other food sources. By the time a child is two years old, he or she possesses a full complement of microbes. "There are more bacteria in your gut than the total number of cells in your body," Roberts says laughingly.

Probiotic bacteria form colonies in the gut and join the existing intestinal microflora. In a healthy gut, these friendly bacteria compete with the disease-causing variety and outnumber them. The goal of consuming probiotics is to reinforce this health-promoting imbalance.

"Generally speaking, probiotics are helpful in maintaining a healthy colon and digestive health," Roberts notes. "They may prevent colon cancer, and can also reduce serum cholesterol levels, as they possess enzymes that can break cholesterol down." They also contain the enzyme lactase, responsible for breaking down lactose, which makes dairy products that contain them a suitable option for individuals with lactose intolerance.

Another important effect of probiotic foods is the prevention and cure of diarrhea, especially that induced after a long dose of antibiotics or by rotaviruses. "One of our goals is to prove by clinical studies in children one to three years old that taking a probiotic strain of bacteria after a dose of antibiotics will prevent diarrhea," states Roberts.

Finally, probiotics are currently being used or tested for alleviating inflammatory bowel disorder and Crohn's disease, and for preventing dental caries, to name a few conditions, even though their exact mechanism is still poorly understood.

"It is the strain of bacteria that determines the beneficial effect," Roberts stresses. Typically, for example, yogurt is made by introducing two common strains—Lactobacillus bulgaricus and Streptococcus thermophilus—into milk. By themselves, these strains are not considered probiotic as they are destroyed by the time they reach the gut. But when other strains like Lactobacillus acidophilus or Lactobacillus caseii, Lactobacillus reutri or any of the Bifidum genera bacteria are added, yogurt becomes a probiotic food full of "bugs" that remain viable in the gut.

There are several probiotic products now available to the consumer. "Dannon's ACTIVIA contains a bacteria called Bifidobacterium animalis DN 173 010," and has become very popular, says Roberts. While each brand has its own trademark strain or mixture of strains, he notes, "Even closely related strains may have different clinical effects."

Though probiotics are now available in pill form, notes Roberts, "It is better to take them as food in order to get the other benefits of food. These bacteria use complex carbohydrates such as cellulose and lignin which are present in the gut and convert them into useful by-products for our health," he adds.

Currently, researchers are trying to understand how probiotic bacteria function at the molecular level and also working to identify and test "designer" bacterial strains that might confer special benefits. "One of our projects," explains Roberts, "is to study how to increase the survival of bacteria present in food, dietary, and pharmaceutical products so that we can prolong the shelf life of these items."

Robert Roberts, Ph.D., is associate professor of food science in the College of Agricultural Sciences. He can be reached at

Last Updated April 23, 2007