The Why Files The Why Files --

Microbial bliss: Meet the bugs in your gut!

Getting practical with probiotics

A green bottle with a foil cap has a "g" on it with yellow dots symbolizing bacteria traveling through the gut
Japanese scientists told the American Chemical Society that this new yogurt defeats bacteria that cause gastritis and stomach ulcers.

As scientists explore the world of beneficial bacteria, plenty of people are wondering whether and how to take probiotics. Following are some questions, some answers, and some unknowns.

Q: Are probiotics dangerous?

A: Probably not. We've seen no reports of hazards among healthy people, although probiotics are not recommended for people with immune impairments due to infection (such as AIDS), chemotherapy or radiation therapy. For the rest of us, eating foods or taking supplements with proven probiotics seems a no-lose proposition; not a panacea, but a cheap, risk-free step that will probably improve health.

Q: If beneficial bacteria live in my gut, do I need to take probiotics regularly?

A: Yes, because newcomer bacteria do not seem to get a firm foothold in the GI tract. "It takes about 10 to 14 days to colonize the gut with daily supplementation of probiotic," says David Pyne of the Department of Physiology at the Australian Institute of Sport, who has studied the effects of probiotics. "Upon cessation of daily supplements, the gut microflora will generally return to its normal state in about the same amount of time. So regular consumption, particularly in periods of stress like athletic competition ... examinations or extended travel, seems appropriate."

Q: How are probiotics different from conventional medicines?

A: Bacteria won't help you if they die, so they must be stored properly. Because the stomach is intensely acidic, probiotics must be able to traverse it intact on their way to the intestine. Finally, the quality of supplements is not regulated as well as the quality of medicines.

The bacteria are long and tubular
Bifidobacteria are common in the human intestine and often used in probiotics.

Q: Must I mind the different strains of bacteria?

A: Unfortunately, yes. Bacteria have been transforming themselves for billions of years, forming an almost infinite variety of bugs, so two strains of the same species may have different effects. "The benefits of probiotics appear to be species- and strain-specific," says Pyne. "This is one of the difficulties in interpreting much of the current experimental work in the scientific literature. Consumers and practitioners have to be diligent in their choice of probiotic supplementation."

Unfortunately, as Mary Ellen Sanders, executive director of the International Scientific Association for Probiotics and Prebiotics points out, companies often sow confusion by inventing names to improve marketability (see "Recommendations for Probiotic Use" in the bibliography).

Q: Should I take probiotics in pills or foods?

A: Hard to know. Probiotics can be taken in supplement pills, or in many cultured products, such as yogurt, cottage cheese, tempeh and kefir. It's not clear which mechanism is better, says Sanders, who consults to the dairy industry on microbiology. "Often probiotics are incorporated into the production of a food product, but we don't really have a good sense of how important that might be. During the fermentation of milk into a dairy product, they may ... produce organic acids or peptides that contribute to the health effects of a probiotic yogurt, but unfortunately there is no good research to sort out the benefits of a fermented dairy product versus a dry supplement."

Q: If so many types of bacteria live inside me, how many types must I take to recover from antibiotics?

A: You may not need to replace hundreds of different types of bacteria, says Huffnagle of the University of Michigan. "One way we believe that probiotics work is as a kind of air traffic controller. By their presence, they can start a whole cascade of events leading to some groups increasing in number, and other groups decreasing in number. That's what makes them probiotics, as opposed to other bacteria in body that are not harmful, but if you ate them, would not change you much."

Bacteria are ovoid.
Lactococcus lactis is another probiotic bacterium.

Q: The million-dollar question: How do good bacteria defeat bad ones?

A: Many mechanisms are conceivable:

I got here first: If the good bugs attach to binding sites on our intestinal cells, bad guys cannot get a foothold.

I own this restaurant: One bacterium may be best adapted to take advantage of the food web inside the intestine.

I fight back: Bacteria make toxic compounds to fight competitors.

I get help from my host: The human immune system may tolerate indigenous bacteria but attack newcomers.

The most likely explanation involves a combination of effects, Young says. "We can imagine a whole mix; these mechanisms are not mutually exclusive. Once you establish a good community, it should be resistant to perturbance. It's like in ecology, diversity is what makes a healthy grassland or forest. Many people think of gut bugs, in combination with the host, as forming a stable ecosystem."

The bacteria look like giant hunks of cylindrical pasta
Photo: NIH
Many harmless or beneficial strains of Escherichia coli are part of the normal community in the gut.

Q: What else don't we know about probiotics?

A: Plenty. Although there is growing evidence that probiotics can help with many conditions, and that antibiotics can contribute, ironically, to illness, "We still have a lot of things we can't explain," says Huffnagle. "We know that children who take multiple courses of broad spectrum antibiotic are at increased risk of developing asthma, but I can show you examples of children who took that, and never developed asthma, and others who took one course of antibiotic, and did get asthma."

In other words, life is complicated, and that holds for critters as huge as humans and as bitty as bacteria. Health and disease are affected by a welter of factors. In some cases, you can improve your health by taking probiotics to establish an optimal microbial zoo in your intestinal plumbing. In other cases, the only benefits could flow to the supplement company.

Learn more in our bibliography.


Terry Devitt, editor; Nathan Hebert, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

©2022, University of Wisconsin, Board of Regents.