That depends on the nutrients available. Water is one necessity for terran life, but you also need some building material and a source of energy. And obviously

That depends on the nutrients available. Water is one necessity for terran life, but you also need some building material and a source of energy. And obviously absence of anything toxic. In short lot of things need to align. Now, even if there are conditions favorable for our bacteria, the question of competition remains. Hypothetically, the invaders can carve niche in local ecology, as they often do on Earth, or they might utterly fail, as happens even more often on Earth. If there are organisms on Europa already, and that is a big 'if', they would have several billions of years worth of head start adapting to the environment and molding it to their image. Conversely, newly introduced bacteria, already damaged by exposure to deep space and deprived of nutrients for many months or years, would see it for the first time. It isn't given that this experience would work out for them, to say at least.