Both nitrosomonas and nitrobacator are gram neg(-), but just as in the cichlid family, these bacteria are going thru constant reclassification, and science becomes more and more precise, using DNA sequencing (etc etc).
So as to whether they are the only nitrogen/ammonia fixing beniaficials is up in the air.
Can certain meds kill them, of course, but are their funds laying around to determine that kind of research in todays anti science/anti intelligence world, that's an entirely different matter.