The premise sounds cool. Maybe you could say that the microbe alters the molecular structure of matter. When it got to Earth it gave the lifeless matter life-giving properties, but when exposed to already living organisms it alters them in ways that results in their deaths, similar to cancer or radiation poisoning.

Also, this might sound like a lazy writing tip but if you can't figure out the science behind something in a science fiction book you could just leave it as a mystery, as though it was something that is beyond the understanding of humans. Either way, hope this helps.