I know that sand can cause impaction in herptiles, toads may be an exception, but there are some nasty horror stories of beardie impactions due to sand, but beardies are at a higher risk for it due to their poorly designed digestive tract. I don't see how it would be impossible to cause impaction in amphibians though, just because they are so small. I have never personally encountered any instances of amphiban impaction, but I would rather be safe than sorry. There is a reason lifespans are shorter in the wild. Just because it comes from that environment does not mean the conditions are optimum for survival. Bearded dragons come from a sandy (Though, there are a bunch of other types of substrate that they encounter, ex: rocks/soil) environment, and sand is terrible to use for them. As for the fungal/bacterium: I'm not sure if you would be able to see either, but I am not very educated in either of them

I like a healthy debate once in a while =D