"Bad Girls Club" star Natalie Nunn's foot was savagely bitten during a fight with co-star Alyssa "Redd" Carswell ... and the injury was so severe, medics thought she'd been attacked by an animal.
Eyewitnesses tell TMZ ... Natalie and "Redd" were shooting a scene for "BGC 13" May 15th when they got into a violent confrontation ... and during the fight, "Redd" chomped down on Nunn's foot ... tearing the flesh away.
Nunn was taken to a hospital ... where she filed a battery report. Law enforcement sources tell us ... police investigated and turned it over to the L.A. City Attorney -- who rejected it based on insufficient evidence.
We don't know if prosecutors looked at any of the show footage. We know cameras WERE rolling during the fight ... and the footage will air on an upcoming episode.
They're called bad girls for a reason.