> What if a therapeutic bacteriophage (mutates and) attacks symbiotic gut bacteria?
We don't have only one species of bacteria in our gut, and even if it was possible to for it mutate to infect one kind of bacteria in out digestive system it's unlikely (like winning the lottery, a few dozens of times) the phage can can mutations to target all of them. In any case, damages to the human symbiotic bacteria already happens with antibiotics (which damage the entirety of the flora not just a single species).
We don't have only one species of bacteria in our gut, and even if it was possible to for it mutate to infect one kind of bacteria in out digestive system it's unlikely (like winning the lottery, a few dozens of times) the phage can can mutations to target all of them. In any case, damages to the human symbiotic bacteria already happens with antibiotics (which damage the entirety of the flora not just a single species).
> Note that 10% of a human's weight
This sounds really fishy.