Probiotics are live microorganisms that play a key role in intestinal function. They achieve this by helping create a healthful bacterial balance within the hosts gut. Our bodies already contain around 1.5 kilograms of probiotic bacteria. However, these microorganisms also occur in fermented foods such as yogurt, kimchi, miso, and some types of cheese. Recently, much hype has surrounded the presumed health benefits of probiotics. Some of these benefits include aiding digestion, lowering blood pressure, improving cognitive function, and alleviating irritable bowel syndrome. However, are probiotics an unmitigated good? As more and more people begin to consume them, emerging research cautions that probiotics may not work in the same way for everyone, and that some strains of probiotics may not even be safe. Now, a new study offers a critical look at the therapeutic benefits of probiotics. Scientists from the Washington University School of Medicine in St. Louis, MO, have investigated the behavior of a strain of Escherichia coli in the intestines of mice. Gautam Dantas, Ph.D. — a professor of pathology and immunology, molecular microbiology, and biomedical engineering at the university — led the new research.