Bacteria are everywhere. In your mouth, in your bed, at the bottom of the sea, underground, anywhere you can name that isn't really hot, really acidic or otherwise too hostile for life. But how did they get there? Some animals are found all over the place but they have stuff like wings and legs and Double Decker buses, bacteria don't have any of that!
On a small scale, they can kind of swim along with big floppy oars/propellers called flagellae, but I'm thinking bigger than that. Of course in the sea they can just float around on the current hoping they don't get eaten by whales, but on land? How did they get to Australia? Or Hawaii?
The key to this is partly their numbers. Being microscopic is less of a problem when there's billions of you, so while individual cells won't make the trip up a mountain or even across the floor the colony as a whole might spread there just by multiplying. Plus, due to their impressive rate of reproduction, even single cells getting to a new place can lead to it being colonised. Bacteria on migrating birds for example can be transferred to new lands. But one of my favourite routes of global bacterial transmission is via the clouds!
Clouds are full of bacteria. In fact, it's been shown that above about four degrees Celsius dust particles can't gather water vapour to form cloud particles, and bacteria are much more likely culprits. Just like marine bacteria float on ocean currents, airborne cloud bacteria can be carried all over the world!
I used to work in a lab looking for thermophilic bacteria in soil samples from the UK. These bugs were needing temperatures of sixty to seventy degrees, so why were they in cold British soil? There's a weather phenomenon in Britain and Europe where everything gets covered in sand blown up from the Sahara desert, and the same winds can carry thermophiles to distant lands! I was working with Geobacillus, which can hibernate in spores for a huge amount of time, so they just kept getting added to the soil by the wind and rain. They were just sitting there for me to find!
Don't worry too much though: any bacteria in the rain are in too small numbers to cause you any harm. After all, most bacteria are friendly!