Could alien bacteria hurt humans? Probably -- they could reproduce on their own, and might be toxic to Earth life. But would they be able to reproduce in a human body, given the raw materials present? Only if the same amino acids were used in the alien bacteria as in the human -- no one knows how likely this would be, as no one has ever seen alien life. Also, all amino acids have a certain "handedness" to them; they are distinct from their mirror-image. If the alien bacteria turned out to be a mirror-image of Earth life, there might be nothing to worry about; only 1 of 20 amino acids is mirror-image symmetric.