Good bacteria should make up about 90% of a healthy person’s gut microbiota, but stress, diet or medications can reduce their numbers. Dr. Robbyn Sockolow, a gastroenterologist at New York-Presbyterian/Weill Cornell Medical Center, said choosing the right probiotic to help boost healthy gut bacteria means paying attention to the genus, species and strain, as well as looking at the number of organisms per dose, how to store the product, shelf life and third-party certifications. The Daily Beast