In terms of extraterrestrial bacteria evolved to be diseases of higher organisms, they are unlikely to be effective against Earth life.
Bacteria evolved to live on specific substrates might not find a usable food.
Even a bacteria that uses a specific toxin to kill its host might not be able to replicate enough in an Earth host to create a useful amount of toxin.
Essentially, a bacterium evolved away from the Earth would likely struggle to survive on earth.