Hmm..a dilemma nonetheless. If you ask me, I wouldn't feed it, but if you asked katt, she would. It's not about bacteria or anything but just the fact that some of the proteins may have deteriorated and thus the quality of food is lowered. In other words, a fresh-killed prey item is better than, a f/t prey item, which is better than a rethawed prey item.
As far as bacteria goes. It all depends if your snake has been fed rethawed food or not. Like BW mentioned, we can't totally compare the wild snakes (in this instance regarding food). Wild snakes can and DO eat carrion. In fact, many species do this. It's all about conditioning. If the snake's bacterial fauna in it's gut is able to cope, then it wouldn't be a problem, if the snake's only used to being fed fresh stuff all it's life, the bacteria may not be able to cope with digesting it.
My suggestion is, if you want to try feeding it, feed it to an adult snake first and watch it's feces. Feed a smaller than normal amount too. Say if you normally feed a corn and adult, feed it a hopper. If it digests it fine then go for it, if the corn regurges or has diarrhea (or funky looking poop) then chuck it.
BW, how much did you feed your water, and how badly rotten was this food item? Only cos, in the wild I've noticed them to eat mostly carrion and inverts (molluscs, crustaceans). This is via personal observation. That said, I don't think captive salvator has the same capabilites of its wild cousin, but I would imagine it would still be able to digest decomposing food to a certain extent.
The Herp Room
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