Don't eat the white snow!

A 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.

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Protip: Yankees can't drive in snow either

Any time it snows in the south, you can count on a few specific things happening. First, everyone goes to the grocery store to buy milk and bread (you know, for milk sandwiches). Second, your transplanted northern neighbor or friend laughs at that phenomenon (which is justified) and claims that because he's from the north, he "knows how to drive in snow." I tell you now, that man is lying. He lies without malice, to be sure—he honestly believes what he just told you. But he does not know how to drive in snow, and most especially he cannot drive safely in snow in the south.

As you probably know, there was a great big snowstorm in the northeast today. Doubtless you are imagining that plows rolled out like clockwork to clear the streets, but that's not what happened. Instead, as the snow fell, almost all the schools and businesses let out early simultaneously, resulting in what I always think of as the Raleigh Effect, a case of traffic paralysis resulting from an seemingly manageable winter weather event. The source of the problem in this case? Yankees who thought they could drive on snow. The result? Well, traffic was barely moving on South Street. Ralph's friend spent two hours going one block. And this is in Waltham MA.

The truth is that Yankees can't drive on snow because they almost never get a chance to learn how. The northeastern states all have very capable infrastructure that usually clears ice and snow off the roads fairly promptly. When that doesn't happen, in this case because plows can't clear roads that are full of cars, Yankee drivers are just as helpless as the rest of us. Your Yankee friend probably does know how to drive on roads that are a bit slushy, but he's almost certainly not equipped to drive on snowy roads in the South, where snowplows and salt trucks are about as common as an egg cream. Do not let him get in his car; he is in danger.

Now I know what you're thinking: "That may be true of citified Yankees, but what about those who come from the rural regions? They probably know how to drive on the powder." And you're right—New Englanders who don't come from the city are much more likely to be able to drive on actual snow. However, you must not let them drive on snow in the South! They are almost certain to wreck, though not out of any fault of their own. You see, these proud country men and women will get into their vehicles, drive confidently out onto the snowy streets, and promptly be hit by some jackass Southerner who thinks his 4 wheel drive means he knows how to drive in snow. Unless they're lucky, in which case they'll get hit by the southerner who thought his Honda was light enough to stay on top of the snow.

This is the painful truth your friend has not yet learned: Most likely, he actually doesn't know how to drive on a snowy road. Even if he does, this amounts only to overconfidence that will result in an accident due to the inevitable chaos of Southerners (who know even less about driving on snow than city Yankees) trying to get to the store because they've run out of fixin's for their milk sandwiches. Dissuade that man from getting in his car if you can... driving on snow, especially in the South, will only result in dents, dings, or a totaled car.

Posts the next couple of weeks will be sporadic as I get some travel in around the holidays. I shall return with more commentary in the new year.

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