I'm always intrigued by this. How big a rock would need to impact to throw a chunk all the way up Earth's gravity well? And does escape velocity create the same kind of atmospheric friction problems that re-entry does (so it'll need to be a big chunk or the bacterium needs to resist extreme heat)?

The whole chain of "really big rock -> chunk of Earth leaving the gravity well -> getting into some kind of stable orbit where it can be pulled into Mars -> surviving re-entry on Mars" doesn't seem like it would happen enough times to create the kind of odds of success that would be needed for a bacterium to successfully colonise Mars.