Can raindrops contain bacteria? And if so, how?
With the uBiome HQ being in San Francisco, we’re well aware of living in a place that needs all the rain it can get.
After all, California is entering its sixth year of drought, and it perhaps explains why people in this part of the world were excited by last month’s (October’s) two inches of rainfall, more than double the historic average.
In a full year, San Francisco generally expects around 24 inches of rain.
Frankly, though, that’s a mere drop in the ocean compared to some other parts of the world.
Consider, for instance, what it would be like to live in Earth’s wettest place—Mawsynram in India—which, on average, gets 11.8 metres of rainfall a year.
11.8 metres is well over 38 feet.
Yup, 38 feet.
Somewhere between the height of a three-story and four-story building. (Wonder if 5th floor apartments go for a premium price?)
All this talk of rain predictably got us thinking about bacteria.
We love the stuff, of course. It’s what we do.
So, are there bacteria in rain?
And is rainwater safe to drink?
Well, starting with the second question, the safety of rainwater rather depends on where you live.
If you’re far away from possible pollution sources, most of the rain you receive will have started life from evaporated seawater, so it’s probably relatively clean.
But if you live in a more built-up area, maybe near to heavy industry or a power station, your rain could contain all kinds of chemicals in the form of particulates, carcinogens, dioxins, and even heavy metals.
Yikes. You probably don’t want to be knocking back that kind of water.
On the subject of bacteria in rainwater, though, it is actually quite common.
Among many others, you’re likely to encounter a species named Pseudomonas syringae.
Let’s take a closer look at it.
P. syringae is a common pathogen found on woody plants such as lilacs, and on a range of trees, including some maples, apple, pear, cherry, plum, and poplar, where it causes blights, cankers, and diebacks.
Curiously, however, these bacteria have a tendency to travel from plants up into the clouds.
It’s believed this occurs through a mechanism known as convective transport, somewhat similar to the “thermals” used by gliders.
P. syringae appear to use clouds as a way of transporting themselves from one place to another.
When they’re up in the clouds, they have a peculiar tendency to cause a phenomenon known as ice nucleation, when chemicals on the surface of the bacterium lead to the formation of tiny ice crystals.
This in turn causes the combination of bacterium and ice to become heavier, making it fall to earth.
The ice thaws as it falls, so we end up with raindrops containing P. syringae.
But not great, because it spreads the pathogen from one location to another.
On the positive side, however, scientists have put this effect to good use in parts of the world where artificial snow is needed.
For example, at the 2010 Winter Olympics in Vancouver, freeze-dried P. syringae was sprayed on the slopes, enabling the accompanying water to freeze at a higher temperature.
Someone who knows a lot about this process is Ulrik Smith Korsholm of the Danish Meteorological Service, who explains that people think of water as always freezing at 0 degrees Celsius, whereas very pure water only freezes at around -40 degrees.
Dust—or other microscopic particles—raises the freezing point to 0 degrees.
While we’re still on the subject of rain, we’re sure you’re familiar with the distinctive smell that’s often apparent after rainfall, especially in the spring.
If you’d like to impress your friends, it may be worth learning that the odor actually has a name – petrichor.
And yup, there’s a bacterial connection to this too.
This time, though, it’s bacteria in soil that causes it, not in the rain itself.
You see, there are two types of bacteria, named Actinomyces and Streptomyces, that grow in soil when conditions are warm and damp.
As the soil dries, however, the bacteria produce spores that lay dormant until it rains again.
The wetness and force of the rainfall causes these spores to be kicked up into the air, releasing a chemical known as geosmin in the form of an aerosol, which readily rises to nose height.
The human nose is particularly sensitive to geosmin, with some individuals being able to detect it at concentrations as low as five parts per trillion.
So it’s what apparently gives freshly fallen rain its distinctive aroma.
Finally, returning to Pseudomonas syringae, you may be curious to hear how it got such an odd name.
The syringae part came about after it was first isolated in 1902 from a diseased lilac, lilac’s scientific name being Syringa vulgaris.
Syringa comes from the ancient Greek “syrinx,” meaning “pipe” or “tube,” referring to lilac’s hollow branches. (Vulgaris, by the way, means “common” or “ordinary,” coming from the Latin word vulgus which means “crowd.”)
As for the Pseudomonas portion of its name, it seems that even the CDC is bemused.
After all, pseudo comes from the Greek word meaning “false,” while monas means “unit.”
As the CDC says, “false unit” seems to make little sense.
So where did this strange name come from?
Well, it was given to the genus in 1894 by German botanist Walter Migula, who, it’s thought, never actually clarified the etymology.
Apparently, he simply believed his newly-isolated bacteria were rather similar to an existing bacterium known as Monas, so could have come up with a name which to him, meant “kind of, but not, Monas.”
Well, it’s a theory.
It is perhaps appropriate, however, that all this talk of rainfall has concluded with a drop of Walter.