...and wait...and wait...and wait.
No biologist will ever likely say 'zero' chance of anything. The question is not of absolute safety--doesn't exist. The question is relative safety to other possible treatments correlated with success and probable side-effects.
Given that the vast majority of the human body is made of bacteria, and we are far from a single organism, adding an additional symbiote is nature's most usual way of adding features...more so than genetic changes.