I am just going to point out that a proper culture for an RI in a snake is done with an endotracheal wash. You can’t get a proper sample just from the mouth in most cases, and the fact that they prescribed a course of injections after a negative test tells me this vet isn’t the best or smartest in the world.
I have a whole post on my experience with my sick snake that was sent by a breeder. I will paste it here because typing all of that out again would be a huge pain.