Microbiologists Brent Christner at Louisiana State University and his colleagues sampled snow from various parts of the world and found this microbe, Pseudomonas syringae, over and over again.
What's interesting is that P. syringae has proteins in their cell wall that bind water, and can catalyze the formation of ice lattice:
Microbes may be responsible for snow—and rain for that matter. They are certainly involved in much of the man-made snow that ski resorts use to cover for Mother Nature's winter lapses. Microorganisms, particularly bacteria, produce proteins in their cell walls that bind water—even if they are dead. In fact, they bind water in such an orderly fashion that water droplets freezing around a microbe almost mirror the natural lattice formation of ice. As a result, bacteria can help snow form at warmer temperatures than would otherwise be the case, which explains why some ski resorts add dead microbes to the mix in their snowmaking machines. And now scientists have discovered such biological precipitation catalysts in natural snows—in such far-flung locations as Montana and Antarctica.
The fun speculation is that microbes like P. syringae are actually nature's way of making snow: Link