Bacteria need to compete for resources, usually when a new bacterial strain is introduced to an environment where there's already a ton of different bacteria, t

Bacteria need to compete for resources, usually when a new bacterial strain is introduced to an environment where there's already a ton of different bacteria, the ones that are already there have adapted to live in balance with the nutrients available and the other members of the community. A new bacterial strain must either find an open niche in that community or somehow outcompete what's already there. The ones already there generally outnumber newcomers to a significant extent. There is no "good" or "bad" in nature, those are words that describe the human perspective and there aren't necessarily bacteria that could come in with small numbers and somehow wipe out an existing population. Its all about competition and resources