Others have answered this fine, but I wanted to add:
There aren’t “good” bacteria or “bad bacteria.” There are just bacteria, some that have greater or lesser potential to cause disease. Bacteria can respond to their environment and can “decide” to peacefully coexist with you or to attack you, depending on what strategy they determine is more likely to keep them happy and fed.
Eg, many, many people are colonized with MRSA which can cause serious disease but usually doesn’t. Part of your immune system’s job is to manage all the different species to kill bad actors and to placate the rest so you have a happy, functional gut.
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