The trillions of bacteria and yeast found in the intestinal tract are one of the hottest topics in medicine. So it's no surprise that probiotics, and foods or supplements containing live organisms that can help maintain a normal balance of good and bad bacteria in the gut, have also gained more attention.

"There's been a tremendous increase in interest in probiotics among practicing physicians and the general public," said Dr. Allan Walker, a professor of pediatrics at Harvard Medical School and an investigator at the Mucosal Immunology and Biology Research Center at MassGeneral Hospital for Children in Boston.

"Over the last 10 to 15 years, research into probiotics and intestinal microbes has taken off, and many talented researchers have entered the field," Walker said. [5 Ways Gut Bacteria Affect Your Health]

In his own research, Walker studies the use of probiotics in infants, and has also co-chaired the Yale Workshop, a group of experts who analyzed the scientific data and published recommendations for physicians on probiotic use in 2011. The group will meet again to review the latest evidence and release updated guidelines in March 2015.

Although the interest in probiotics is skyrocketing, the medical community is not as excited about probiotics as the public, because doctors want to have strong scientific evidence before making recommendations for patients, Walker told Live Science. What's really needed to move the science ahead is to gather more evidence from multiple-center clinical trials, he said.

So far, results from studies have been mixed, with some researchers finding that probiotics are beneficial for health and others finding no benefit. There are four conditions that Walker said currently have the strongest positive evidence to support their use.

Antibiotic-associated diarrhea

There's solid evidence, in both children and adults, that giving probiotics to people when they start on a course of antibiotics may help prevent diarrhea, a common side effect of taking these drugs. Antibiotics can wipe out both good and bad bacteria in the gut, disturbing the normal intestinal balance and resulting in diarrhea in as many as 30 percent of people receiving treatment.

Probiotics can help replenish the good strains of bacteria and restore intestinal balance.

"It takes a long time for the intestines to recover from taking antibiotics," Walker said. For this reason, he suggests that patients continue on probiotics for a month after they finish taking antibiotics.

Two of the best-studied probiotic strains for antibiotic-associated diarrhea are the yeast Saccharomyces boulardii and Lactobacillus GG.

Infectious childhood diarrhea

There's solid data supporting the use of probiotics to help treat infectious diarrhea in young children, especially diarrhea caused by rotavirus, a bug spread easily in babies and young children, Walker said.

Rotavirus is a common infection among children attending day care that can quickly spread when toys, surfaces and hands become contaminated with the virus. Infected children may have severe diarrhea, fever and stomach pain, and are at risk for becoming dehydrated.

Research has found that when probiotics are used as a treatment to ease symptoms of the virus, they may shorten the duration of diarrhea and its frequency by about a day.

Lactobacillus GG is the best-studied strain.

Inflammatory bowel disease (IBD)

Ulcerative colitis, one type of inflammatory bowel disease that affects the lining of the large intestine, appears to be more responsive to probiotics than the other main type of inflammatory bowel disease, Crohn's disease. A flare-up of ulcerative colitis, an autoimmune disease, produces symptoms including frequent, and often bloody, diarrhea; abdominal pain and cramping; and weight loss.

The strongest evidence for the use of probiotics in ulcerative colitis shows that probiotics should be taken once a person has been brought into remission, Walker said. Research has found that people who take probiotics at that point can sustain remission for longer, perhaps for as much as six months to one year, compared with individuals not taking a probiotic, he said.

For people with ulcerative colitis, probiotics seem to work better behind-the-scenes, almost as a preventive measure, rather than as a treatment taken during the inflammatory, flare-up phase of the condition, Walker said. The most effective strains are Escherichia coli Nissle (which is not a harmful strain of E. coli), and VSL#3, which is a probiotic containing eight bacterial strains.

Pouchitis

This condition involves inflammation of the lining of the intestines, and can occur following surgery to remove large portions of the colon in people with severe ulcerative colitis. Pouchitis can affect up to 50 percent of people who undergo this surgery, which replaces the diseased colon by creating an internal pouch with the ileum, the lowest portion of the small intestine.

Like the colon, this pouch can become a reservoir for bacteria, Walker said. To sustain remission of pouchitis, patients are often given a "cocktail of antibiotics," he said.

Studies suggest that taking probiotics following surgery might help prevent pouchitis, and also maintain remission of the disease after treatment with antibiotics. The most helpful strain appears to be VSL#3.

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