Microorganisms in air
Using computer models, researchers from labs in the UK and Switzerland (ref. 1) examined just how far bacteria of various sizes could travel. A global atmospheric circulation model was used to virtually release microbes with diameters of 9, 20, 40 and 60 μm from various places around the globe and see how far they got. Although the paper focuses more on the larger, protist-sized microbes, it should come as no surprise that the tiny ones travelled both further and faster.
The found that although tiny microbes travel widely within the hemisphere in which they are released (i.e the northern or southern hemisphere) the amount of movement between hemispheres is remarkably small. Bacteria released high in the atmosphere in South America, for example, were unlikely to reach Europe but spread easily to Australia. West-to-east dispersal was more prevalent than the opposite, as the bacteria were picked up by wind currents and sailed along with them.
One question of interest this helps to address is how microbial diversity shapes their evolution. If bacteria are so good at travelling, why are they not all the same in every part of the world? Different hypotheses have arisen to answer this, my favourite is that while bacteria can travel with ease, they do not always settle in their new environment. In the words of Laurens Becking "Everything is everywhere, but, the environment selects". A group of bacteria might be able to travel from Mexico to Australia, but they may simply be killed or outcompeted when they get there.
This research certainly supports the view that it is possible for bacteria to travel long distances. The northern/southern hemisphere boundary is an interesting one, but bacteria don't just travel by wind. Water currents, insect or animal vectors, or at the very least stowing away in the luggage (or lungs!) of a jet-setting buisnessman all provide multiple ways for small non-motile organisms to move around the globe. Despite having limited mobility, there is almost no limit to how far bacteria can travel. They've ever been sent into space!
Wilkinson, D., Koumoutsaris, S., Mitchell, E., & Bey, I. (2011). Modelling the effect of size on the aerial dispersal of microorganisms Journal of Biogeography DOI: 10.1111/j.1365-2699.2011.02569.x