I'm not expert on these things, but if these microbes evolved without any animals around, I wouldn't have expected them to evolve any of the characteristics tha

I'm not expert on these things, but if these microbes evolved without any animals around, I wouldn't have expected them to evolve any of the characteristics that make some bacteria harmful. For example, our immune system should be able to handle them easily, because they never had to fight an immune system before. There could be some crazy thing that happens, but I bet the odds are low enough that we don't need to worry too much.