Meteor impacts throwing debris into space can happen for sure, but we're talking about stuff that ends up on the outside of the ISS. The bacteria would have to

Meteor impacts throwing debris into space can happen for sure, but we're talking about stuff that ends up on the outside of the ISS. The bacteria would have to be on something large enough to shelter it from the heat and wind of the original impact and high-speed passage through the atmosphere. It would have to end up in a stable orbit in the ballpark of where the ISS is. Surviving bacteria would then have to make their way out of this object and float around in space. And then their orbit drifts until they hit the ISS, mostly likely at very high speed unless they happen to be in almost exactly the same orbit. Feels pretty unlikely.