Would extraterrestrial bacteria be dangerous to us if we physically came in contact with such?
There was some discussion about it in this thread.
There is no good answer to this question. I'm sure people will say things - almost anything anyone can say is pure opinion.
If you stipulated a bunch of conditions the foreign bacteria had to meet then it would be more fun. IMO: It is probably true that they would have to have a lot in common with extant Earth life in order to be able to function in any way either helpful or deleterious.
What we really need is exobacteria that eat air pollutants and synthesize gasoline as a by product.... preferably exuded into ready-to-use tanks.
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.
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