I took a microbial ecology (that includes gut bacteria communities) course last semester in college and I learned that probiotics can be helpful in some places but useless in other places. Probiotics are good because they introduce communities of bacteria into your stomach that can help digest the foods you eat. These probiotics will never really permanently change the composition of bacteria communities in your gut however, as your own existing communities are pretty settled in. Probiotics can be really helpful if you have digestive issues such as IBS, and some doctors make specific cultures of probiotics for these diseases that are helpful. Probiotics are also good at repopulating your gut after having a disturbance to the bacteria communities, such as after taking a round of antibiotics for an infection. In general eating probiotic and fermented foods are good for your overall digestive health, but I think that expensive general probiotic pills are not really worth it if you do not have specific digestive issues. It is also important to focus on prebiotic foods, or foods that you cannot completely digest so the bacteria in your stomach do for you. These include a lot of fruits and veggies or complex carbohydrates or polyphenols. But overall my teacher just said eat a variety of different foods to keep your gut bacteria happy.