What about extremophiles?
Like this guy for instance:
http://www.spacedaily.com/news/life-99l.html
Because you are a scientist I understand your need for skepticism. It goes with the territory. I share your skepticism as certainly there is no proof that any micro organisms exist on comets, but keep in mind this will be our very first peek. And dust from one comet's tail will not definitely prove one way or another what exists on all the comets in our neck of the universe.
Further, radiation's effects on earth based lifeforms should not necessarily be used to extrapolate effects on non-earth based lifeforms should they exist.