Dirt smells good
May 8, 2020 10:01 AM   Subscribe

Dirt Doesn't Smell like Dirt: It smells like bacteria. But why? (Scientific American)
"The natural history of these bacteria contains a clue. Streptomyces, amazingly for bacteria, behave like fungi. Fungi grow as a mass of branched filaments called mycelium that often sprouts individual reproductive cells called spores. In spite of the bacterial reputation for making tiny individual cells, Streptomyces does this too. It's a case of convergent evolution, just as bats resemble birds and whales resemble fish.
Streptomyces's mycelia can sometimes reach a few centimeters in size and often sprout chains or whorls of spores in a variety of interesting and sometimes complex configurations. But because they live immersed in dirt, dispersing those spores is difficult. Wind and water are unpredictable and unlikely to get you very far. The remaining options comprise miscellaneous creepy crawlies.
So the scientists put two and two together: is geosmin the bacterial equivalent of the smell of ripe fruit?"
posted by not_the_water (16 comments total) 24 users marked this as a favorite
Other than a quibble with the title, I thought that was really interesting. I believe that Geosmin is also responsible for the nice smell you get after it rains.
posted by Carillon at 10:21 AM on May 8

Also, coins don’t actually smell like metal. They smell like 1-octen-2-one, one of the products of reactions when the perspiration and oils on your hands come in contact with the metal in the coins.
posted by Your Childhood Pet Rock at 10:24 AM on May 8 [5 favorites]

A soil scientist friend once posted an article on a village in India where they produce an essential oil from soil called "mitti attar". I don't remember if it was this article. I immediately found some online and bought a vial of it. It smells like very pleasant soil.

I'm always happy when I get a soil sample with springtails in it. They are cute.
posted by acrasis at 10:32 AM on May 8 [6 favorites]

What is the relationship between our intestinal flora and the bacteria in dirt?
posted by anthill at 10:32 AM on May 8

I used to work for a large pharmaceutical company and when I had to travel to the plant that made penicillin that entire area smelled like dirt, too. Mother nature is a maaaad scientist...
posted by JoeZydeco at 11:15 AM on May 8

What a delightful article! Thank you!
posted by Emmy Noether at 11:42 AM on May 8

What is the relationship between our intestinal flora and the bacteria in dirt?

My parents would have insisted that I had a 1:1 match given how often they told me to stop eating dirt as a young child.
posted by wierdo at 12:05 PM on May 8 [3 favorites]

I believe that Geosmin is also responsible for the nice smell you get after it rains.


(One of my favorite descriptor words when write up tasting notes for whiskey at my former place of work.)

posted by Insert Clever Name Here at 12:15 PM on May 8 [2 favorites]

One of the things that annoyed me as a kid is that the smelly crayon or marker for "dirt" was actually leaf litter- I'm assuming it was the petrichor getting released in the air when rain started. Garden dirt didn't smell that way! I guess the fact that the color was more like dead leaves than organic soil should have been a tipoff.
posted by Hermeowne Grangepurr at 1:01 PM on May 8

While I like the smell of geosmin, it admittedly gets to be a bit much when you have liters and liters of Streptomyces cultures and lots of plates, all in an enclosed room. (I actually didn't know about it when I first started to grow these species, and I was quite confused when I walked into lab and found that it smelled of wet earth.)

I'd always idly wondered why they made so much of it, so this was fun to read. Next up: a better explanation for why humans are so sensitive to it!

What is the relationship between our intestinal flora and the bacteria in dirt?

Complicated? Streptomyces strains (discussed here) are generally not present in healthy gut microbiomes. Among other things, they usually need oxygen, which is scarce within the gut. Most gut-dwelling bacteria are anaerobic and do not appreciate all the oxygen present in the top layer of soil (though there are anaerobic soil environments, as in a deep compost heap that is not turned over). But some kinds of bacteria can tolerate low- and high-oxygen environments and so can survive in the gut and in the soil relatively happily. Through all the crazy mechanisms of horizontal gene transfer, that means that sets of genes from soil bacteria can ultimately make it into gut bacteria, and visa versa, so species from the two environments are not necessarily as genetically disconnected as their different requirements might make one think.
posted by ASF Tod und Schwerkraft at 1:57 PM on May 8 [9 favorites]

That was wonderful .

Next up: a better explanation for why humans are so sensitive to it!

Yes! IIRC there’s some reasonably good experimental evidence that it improves mood in most people, not just gardeners.
posted by clew at 3:18 PM on May 8 [2 favorites]

Recently the makers of cat litter added a gel ingredient to absorb the ammonia smell. I acted like casting material, coating one cat's entire lower leg, and I had a hard time getting it off. I grew annoyed with having to spent $50 per month, instead of the usual $13 for stuff that worked, but changed. So, I just started using dirt, in my cat's box. Well surprise, there is no smell of ammonia any more, no smell of anything, but dirt. And my one cat has lost about 7 pounds, probably from whatever reaction he had to the chemicals in cat litter. All is well around here, and the myrtle, where I dump some more lumpy dirt, has thrived beyond belief.
posted by Oyéah at 3:38 PM on May 8 [6 favorites]

Ah, but what is responsible for the different and even better smell of granite dirt with conifers growing in it, either at the side of the ocean or at least halfway up a mountain?
posted by eviemath at 4:05 PM on May 8 [1 favorite]

The first successful antibiotic treatment for TB was isolated from Streptomyces griseus according to the Wikipedia article I just lost the link to — and Bob Dole was among the earliest patients who were cured by it.

It's interesting that your Myrtle is doing so well with the night soil, Oyéah. Myrtecaceae use actinomycetes, of which Streptomyces is one, to fix nitrogen the way legumes use rhizobia.

Myrtlewood bowls often have a desirable black accented marbling which I've always heard is fungal, but I've been wondering whether it could actually be due to the actinomycetes.
posted by jamjam at 6:28 PM on May 8 [3 favorites]

Oh, and I forgot something I read a long time ago to the effect that indigenous peoples of the north west coast of North America used a tea of which myrtle leaves and I think root were principal ingredients to treat TB. I meant to write the author and point out the possible streptomycin connection, but I never got around to it.
posted by jamjam at 6:37 PM on May 8

Jamjam, maybe this was it?
posted by bunderful at 7:46 AM on May 9 [1 favorite]

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