Feb 292008
ResearchBlogging.orgA bunch of really great stuff came out today, so much that I’m not sure I’ll be able to get to all of it. We can start with the shortest article of interest, a paper in Science that’s sure to interest mothers whose children stand around with their tongues out in a flurry. Brent Christner and colleagues have discovered that snow from pretty much anywhere contains bacteria (1). Well sure, you probably suspected that, but these bacteria didn’t move up from the ground… they fell down from the sky.

Crystals generally form more readily when they have a nucleation point. Protein crystallographers are well aware that seeding their trays with microcrystals from a partially successful screen can significantly improve results. Non-scientists can see this as well, in the growth of ice crystals on certain surfaces or the popular home experiment of crystallizing sugar onto a string. The very rough string surface provides many more nucleation points than the smoother bowl surface, or the liquid, and so crystal formation on the string is preferred. A similar principle is at work in the formation of ice crystals in clouds. Any small particle can conceivably serve as a nucleation point: aerosols, dust, and even living things. And of course, once they form, those ice crystals can return to earth as snow or rain.

The authors aseptically collected fresh snow samples from various locations (France, Montana, the Yukon, and Antarctica) and melted them. They filtered the snowmelt and resuspended the filtrate in a smaller volume. They then assayed the freezing temperature between -2 and -9 °C. To prove that the ice nuclei (IN) were biological in origin they attempted to inactivate them using heat (to denature proteins) and lysozyme (to break up bacterial membranes). The experiment is pretty simple—in fact, except for the parts that explicitly prove bacteria are involved you could mimic this at home. The next part isn’t so easy to try yourself: they used flow cytometry to identify particles in the snow containing DNA.

DNA-containing particles were found at all locations, as were IN that could be inactivated by heat. Several locations also had IN that could be inactivated by lysozyme. The authors chalk up the apparent discrepancy to incomplete hydrolysis or lysozyme resistance. The authors don’t explicitly identify the bacteria in the article, but some of the points they make suggest that they may be species that are commensal or pathogenic towards plants. Of course there’s no guarantee that the bacteria that land this way are still alive: the authors didn’t assay motility or attempt to culture from the isolated IN. That lysozyme had an effect, however, suggests that the cells did not crack just from the conditions, and everyone who works in the biological sciences knows that even humdrum everyday bacteria like E. coli can survive freezing. The obvious implication is that it will be very difficult to control the spread of any species that can pull off the same trick. All they have to do is get blown up high enough and come down in the next rainstorm.

In spite of this news, it’s probably not dangerous for your kids to eat a few snowflakes from the sky. This finding isn’t so much a revelation as it is a reminder: bacteria are all around us and coat essentially everything we touch, eat, or drink. And that’s true even if you don’t drink anything but whisky and rainwater.

1. Christner, B.C., Morris, C.E., Foreman, C.M., Cai, R., Sands, D.C. (2008). Ubiquity of Biological Ice Nucleators in Snowfall. Science, 319(5867), 1214-1214. DOI: 10.1126/science.1149757

POSTSCRIPT: Tara Smith has good post on this, with a forum image for all you 4channers. Ed Yong also has a great post.

 Posted by at 1:34 PM

  One Response to “Don’t eat the white snow!”

  1. What if I just drink whiskey? Can I be free of the dreaded bacteria?!

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